KVS: Use `tezt-bam` for the PBT test and fix a deadlock issue

Stacked on !15148 (merged)

What

Replace the PBT test with QCheck2 by tezt-bam engine and fix a deadlock issue.

Why

While debugging a memory leak in the KVS, I was not able to pin down the issue with QCheck2. tezt-bam was helpful for that. By moving from QCheck2 to tezt-bam, we realize quickly that a deadlock can arise. The second commit fixes it.

How

Nothing special, just using tezt-bam. The fix for the deadlock is detailed in comment.

Manually testing the MR

dune exec src/lib_stdlib_unix/test/main.exe -- kvs -v --loop

Observe it works. Observe it is stuck if you remove the last commit.