mempool: Recheck improvements and fixes

[hvanz]

Tracking issue for improvements and fixes to the rechecking mechanism in the mempool.

Bugs:

• [x] #1827
• [x] #2225
• [ ] #2907

Potential improvements

• [X] #895
• [ ] ~~Explore batching transactions when rechecking. One ABCI request means locking the app only once.~~
• [ ] ~~Explore supporting flexible periodic rechecking. That is, instead of running recheck after every committed block, allow running it every N>1 blocks.~~