Adjust how IStore deals with state references and tx nonce

[dahlia]

This consists of two changes on the IStore interface and its implementations:

• Tx nonces is no more stacked but encoded as a single 64-bit integer instead.
• Multiple state references for an address can be fetched at a time, without multiple method calls.

These changes are needed for implementing #272.

[codecov]

Codecov Report

Merging #307 into master will decrease coverage by 0.2%.
The diff coverage is 90.47%.

@@            Coverage Diff             @@
##           master     #307      +/-   ##
==========================================
- Coverage   87.51%   87.31%   -0.21%     
==========================================
  Files         187      189       +2     
  Lines       12690    12604      -86     
==========================================
- Hits        11106    11005     -101     
- Misses       1335     1343       +8     
- Partials      249      256       +7

Impacted Files	Coverage Δ
Libplanet/Store/BaseStore.cs	100% <ø> (ø)	⬆️
Libplanet.Tests/Blockchain/BlockChainTest.cs	98.59% <100%> (ø)	⬆️
Libplanet.Tests/Store/StoreTracker.cs	54.31% <100%> (+2.24%)	⬆️
Libplanet.Tests/Store/StoreTest.cs	98.57% <100%> (-0.15%)	⬇️
Libplanet.Tests/Store/StoreExtensionTest.cs	100% <100%> (ø)
Libplanet/Store/LiteDBStore.cs	92.62% <75%> (+0.23%)	⬆️
Libplanet/Blockchain/BlockChain.cs	95.52% <82.35%> (-1.04%)	⬇️
Libplanet/Store/FileStore.cs	90.42% <84%> (-2.99%)	⬇️
Libplanet/Store/StoreExtension.cs	85.71% <85.71%> (ø)
Libplanet.Tests/Store/StoreFixture.cs	92.04% <0%> (-4.55%)	⬇️
... and 3 more

[dahlia]

@earlbread @longfin I fixed a bug that tests didn't work on Unity (I had to replace ValueTuple with Tuple), and wrote a docs about how to run Unity tests. Please review this again.