Properly fuzz concurrency behaviors

[Tpt]

Currently the fuzzers only write and read using a single thread and do not control the background threads concurrency. It might be nice to use a tool like loom to properly cover possible concurrency issues.

It should also make the debugging of bugs like #124 easier (no more reproduction issues).

[cheme]

A bit unrelated, but fuzzing can be more reproducible by using EnableCommitPipelineStages to target some state specifically (does not use threads IIRC), it is far from perfect but can be useful (IIRC there was a state I did not manage to stop on, I think when commit payload is in the log overlay and unflushed).

[Tpt]

With #165 I believe we have a fairly good base for concurrency testing: insertion and deletions are covered from commit to log clean with both btree and hash and reference counting and not. Btree iteration is also tested.