Improve re-insertion of TXs from disconnected blocks into mempool

[pinheadmz]

Closes #306

Port of bcoin-org/bcoin#917, although it hasn't been reviewed or merged yet over there. I think this is a bigger concern for Handshake since this codebase will represent 100% of the network, and chain reorgs are more likely on hsd in the early days.

The contextual checks for TXs with covenants re-entering mempool after a block disconnect has already been written in _handleReorg(), so for that case I just added a test.

The other cases ported over from bcoin are premature coinbase spends, and premature BIP68 sequence lock spends.

With these contextual checks in place, the "don't remove block if mempool empty" check can be safely removed.

[codecov-io]

Codecov Report

Merging #319 into master will increase coverage by 0.71%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #319      +/-   ##
==========================================
+ Coverage   58.68%   59.39%   +0.71%     
==========================================
  Files         129      129              
  Lines       35795    35793       -2     
  Branches     6027     6027              
==========================================
+ Hits        21007    21261     +254     
+ Misses      14788    14532     -256

Impacted Files	Coverage Δ
lib/mempool/mempool.js	52.56% <100%> (+9.73%)	⬆️
lib/blockchain/chain.js	60.75% <0%> (+0.3%)	⬆️
lib/covenants/namestate.js	87.37% <0%> (+0.4%)	⬆️
lib/blockchain/chaindb.js	64.87% <0%> (+1.03%)	⬆️
lib/covenants/bitfield.js	89.53% <0%> (+1.16%)	⬆️
lib/covenants/namedelta.js	87.06% <0%> (+1.99%)	⬆️
lib/primitives/covenant.js	72.01% <0%> (+2.29%)	⬆️
lib/utils/binary.js	58.97% <0%> (+2.56%)	⬆️
lib/mempool/contractstate.js	80.09% <0%> (+4.97%)	⬆️
... and 2 more

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[pinheadmz]

Also thinking we can deal with Claims and AirdropProofs here too:

hsd/lib/mempool/mempool.js

Lines 414 to 416 in c554a6f

	this.dropClaims();
	this.dropAirdrops();
	this.contracts.clear();

They can be re-evaluated on reorgs. Claims for example commit to a block hash so that needs to be re-checked.

[pinheadmz]

Ok this latest commit adds the Claims and AirDrops back into the mempool after a reorg. This will require diligent review if we want to keep it. From what I can tell, there are no restrictions on AirDrops if the chain is rewound (unlike premature coinbase spends). The only restriction on Claims would be the inception time (RRSIG contains a timestamp which is compared to the chain tip) so I added that exception.

[pinheadmz]

Forgot to include checks for block commitment of Claims re-entering mempool in last commit, that has been updated. Everything should be covered now.

[pinheadmz]

Bonus TODO: this is getting pretty advanced, but Bitcoin Core will even hold on to the evicted TXs until after a reorg is complete, and then see if any are still ok (for example, a coinbase spend might still be valid after a reorg, just not during):

https://github.com/bitcoin/bitcoin/blob/631df3ee87ec93e1fc748715671cdb5cff7308e6/src/txmempool.h#L766-L776

[chjj]

After looking at this for a while, I can't find anything wrong with it. Merging for now.