Backport priority mempool fixes from v0.35.x to v0.34.x. (#8962)

This is a manual backport of the changes from these commits:

- bc49f66 Add more unit tests for the priority mempool. (#8961)
- 9b02094 Fix unbounded heap growth in the priority mempool. (#8944)

Imports and type signatures have been updated to match the v0.34 usage.

CHANGELOG_PENDING.md

@@ -2,7 +2,7 @@
 
 ## v0.34.20
 
-Special thanks to external contributors on this release:
+Special thanks to external contributors on this release: @yihuang
 
 Friendly reminder, we have a [bug bounty program](https://hackerone.com/tendermint).
 
@@ -24,5 +24,6 @@ Friendly reminder, we have a [bug bounty program](https://hackerone.com/tendermi
 
 ### BUG FIXES
 
-- [blocksync] [\#8496](https://github.com/tendermint/tendermint/pull/8496) validate block against state before persisting it to disk (@cmwaters)
-- (indexer) [#8625](https://github.com/tendermint/tendermint/pull/8625) Fix overriding tx index of duplicated txs.
+- [blocksync] [#8496](https://github.com/tendermint/tendermint/pull/8496) validate block against state before persisting it to disk. (@cmwaters)
+- [indexer] [#8625](https://github.com/tendermint/tendermint/pull/8625) Fix overriding tx index of duplicated txs. (@yihuang)
+- [mempool] \#8962 Backport priority mempool fixes from v0.35.x to v0.34.x (@creachdair).