Mempool: evict expired transactions regardless of current mempool size

[pinheadmz]

Port of bcoin-org/bcoin#997 with explanation here

(see also #514 )

Addresses #430 -- solution 4

Summary: We already have logic to remove "old" transactions from the mempool, but that logic is never executed unless the mempool is full. This PR checks for and evicts old transactions every time a new transaction is added to the mempool.

[coveralls]

Pull Request Test Coverage Report for Build 480586213

• 9 of 9 (100.0%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage increased (+0.2%) to 59.597%

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Files with Coverage Reduction	New Missed Lines	%
lib/covenants/rules.js	1	67.45%

Totals
Change from base Build 480546966:	0.2%
Covered Lines:	19509
Relevant Lines:	30510

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💛 - Coveralls

[chjj]

Is there perf overhead here? Inserting everything into a heap is going to be costly.

[pinheadmz]

Is there perf overhead here? Inserting everything into a heap is going to be costly.

@chjj thanks for taking a look, I refactored the patch so we just screen and evict expired mempool entries before getting to the Heap, but still before checking overall mempool size for excess and returning false.