Replace research accrual calculations with superblock snapshots

[cyrossignol]

Current research reward accrual calculations use a two- or three-point magnitude average to determine the research reward owed to a CPID. This rudimentary approach is simple and inexpensive to compute, but it's not very accurate. Unclaimed research rewards can fluctuate as a participant's magnitude or the network magnitude unit changes.

These updates replace the old implementation with a strategy that calculates accrual precisely from the magnitude values in each superblock. The system takes a snapshot of the pending research reward accrued for each CPID whenever the network produces a new superblock. When a CPID stakes a block, any accrual earned since the current superblock is added to the accrual snapshot and submitted as the research reward claimed for the block. By using snapshots, we avoid the performance penalty incurred by scanning the chain for each new block to determine accrual dynamically (which could require reading hundreds of MB of blocks from disk as the network grows). Instead, accrual calculations are essentially instantaneous. It's a rendition of the "superblock windows" concept described in #768.

The wallet caches accrual snapshots on disk as flat files in the data directory and reloads the latest snapshot on startup or when reorganizing a section of the chain that contains a superblock. For now, we'll need to include the "accrual" directory in the official blockchain snapshot, but we can add a way to regenerate this data in the future. [Note: this was implemented, along with snapshot integrity checking in #1727.]

These changes include a new IAccrualComputer (#1583) implementation used for snapshot accrual in version 11 blocks that follows different rules than the old "research age" computer. In particular, it:

• pins the network magnitude unit to a static value of 0.25 (Please consider removing the magnitude unit #1578)
• raises the max research reward allowed per block to 16384 GRC (2 day's accrual at max mag) (Research earning cap of 12750GRC #913)
• removes the 500 GRC research reward limit for a newbie's first block
• removes the 6-month expiration for unclaimed rewards (assuming valid beacon) (Inconsistent definitions of time intervals #1579)
• removes the 10-block spacing requirement between research reward claims for the same CPID
• removes the 5-day payment-per-day limit based on lifetime average magnitude
• removes legacy tolerances for floating-point error and time drift in old tally code (Numerous Tolerances allow staking up to 25% more than Fair #370)

The new system bootstraps itself when the first version 11 block arrives at the mandatory threshold and creates the first snapshot from about 6 months of the preceding superblocks. For a more seamless transition, the initial look-back depth matches the depth allowed for the current system, but research rewards will accrue indefinitely after the switch. Participants still need to renew beacons every 6 months.

[jamescowens]

Initial review looks really good with the exception of the couple of comments I made. I am going to go ahead and run this on my testnet nodes.

[jamescowens]

@barton2526 You are added for your opinion on the rules simplication v11+ that @cyrossignol referred to in his write-up above. Other folks may want to comment too. I will put up the write-up in the #development channel.

[jamescowens]

@cyrossignol. The travis builds are failing for MacOS. Clang is picky....

CXX test/test_test_gridcoin-netbase_tests.o
CXX test/neuralnet/test_test_gridcoin-claim_tests.o
CXX test/neuralnet/test_test_gridcoin-cpid_tests.o
CXX test/neuralnet/test_test_gridcoin-magnitude_tests.o
CXX test/neuralnet/test_test_gridcoin-project_tests.o
CXX test/neuralnet/test_test_gridcoin-researcher_tests.o
CXX test/neuralnet/test_test_gridcoin-superblock_tests.o
AR libgridcoin_util.a
/home/travis/build/jamescowens/Gridcoin-Research/depends/x86_64-apple-darwin11/native/bin/x86_64-apple-darwin11-ranlib: file: libgridcoin_util.a(libgridcoin_util_a-scrypt-arm.o) has no symbols
/home/travis/build/jamescowens/Gridcoin-Research/depends/x86_64-apple-darwin11/native/bin/x86_64-apple-darwin11-ranlib: file: libgridcoin_util.a(libgridcoin_util_a-scrypt-x86_64.o) has no symbols
/home/travis/build/jamescowens/Gridcoin-Research/depends/x86_64-apple-darwin11/native/bin/x86_64-apple-darwin11-ranlib: file: libgridcoin_util.a(libgridcoin_util_a-scrypt-x86.o) has no symbols
/home/travis/build/jamescowens/Gridcoin-Research/depends/x86_64-apple-darwin11/native/bin/x86_64-apple-darwin11-ranlib: file: libgridcoin_util.a(libgridcoin_util_a-scrypt-arm.o) has no symbols
/home/travis/build/jamescowens/Gridcoin-Research/depends/x86_64-apple-darwin11/native/bin/x86_64-apple-darwin11-ranlib: file: libgridcoin_util.a(libgridcoin_util_a-scrypt-x86_64.o) has no symbols
/home/travis/build/jamescowens/Gridcoin-Research/depends/x86_64-apple-darwin11/native/bin/x86_64-apple-darwin11-ranlib: file: libgridcoin_util.a(libgridcoin_util_a-scrypt-x86.o) has no symbols
AR crypto/libgridcoin_crypto_base.a
AR leveldb/libleveldb.a
AR leveldb/libmemenv.a
CXXLD gridcoinresearchd
clang: warning: argument unused during compilation: '-pthread'
clang: warning: argument unused during compilation: '-pie'
clang: warning: argument unused during compilation: '-pthread'
Undefined symbols for architecture x86_64:
"(anonymous namespace)::AccrualSnapshot::CURRENT_VERSION", referenced from:
(anonymous namespace)::AccrualSnapshotRepository::Store(unsigned long long, std::__1::unordered_map<NN::Cpid, NN::ResearchAccount, std::__1::hashNN::Cpid, std::__1::equal_toNN::Cpid, std::__1::allocator<std::__1::pair<NN::Cpid const, NN::ResearchAccount> > > const&) in libgridcoin_util.a(libgridcoin_util_a-tally.o)
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
Makefile:2980: recipe for target 'gridcoinresearchd' failed
make[2]: *** [gridcoinresearchd] Error 1
make[2]: *** Waiting for unfinished jobs....
make[2]: Leaving directory '/home/travis/build/jamescowens/Gridcoin-Research/build/gridcoin-x86_64-apple-darwin11/src'
Makefile:8027: recipe for target 'all-recursive' failed
make[1]: *** [all-recursive] Error 1
make[1]: Leaving directory '/home/travis/build/jamescowens/Gridcoin-Research/build/gridcoin-x86_64-apple-darwin11/src'
Makefile:751: recipe for target 'all-recursive' failed
make: *** [all-recursive] Error 1
Build failure. Verbose build follows.
Making all in src