mempool: Disable WAL by default

[melekes]

This should reduce file IO and disk space. For those, who want better guarantees, a note in https://tendermint.com/docs/tendermint-core/running-in-production.html is left of how to enable mempool WAL. In the future, we may want to provide better support and tooling for the mempool WAL recover procedure.

Not sure the measurements below are correct, but that's better than having no pictures 😃 These were taken using iostat -yd 5 while tendermint (kvstore) & tm-bench were running in parallel inside my virtual box.

• Updated all relevant documentation in docs
• Updated all code comments where relevant
• Wrote tests
• Updated CHANGELOG_PENDING.md

[codecov-io]

Codecov Report

Merging #2490 into develop will increase coverage by 0.08%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop    #2490      +/-   ##
===========================================
+ Coverage     61.4%   61.49%   +0.08%     
===========================================
  Files          197      197              
  Lines        16432    16407      -25     
===========================================
- Hits         10090    10089       -1     
+ Misses        5507     5487      -20     
+ Partials       835      831       -4

Impacted Files	Coverage Δ
config/config.go	82.67% <100%> (ø)	⬆️
libs/db/remotedb/remotedb.go	36.84% <0%> (-4.69%)	⬇️
consensus/reactor.go	73.38% <0%> (+0.77%)	⬆️
lite/dbprovider.go	59.18% <0%> (+3.02%)	⬆️