sled pinstore tests deadlock rarely

[koivunej]

When running the common tests for sled pinstore on a loop with a machine more than 8 cores, it seems that they sometimes lock up: while true; do cargo test --lib -- --test-threads 8 repo::kv --nocapture; done.

Looking this deadlock through gdb shows only awaiting threads, so it kinda feels like the wakeup is being missed.

With some additional debugging (a future which printlns poll calls to the flush_async) it'd seem that the lockup only happens whenever all 7/8 tests which flush (cannot_recursively_unpin_unpinned doesn't flush) poll the flush future without before any of them returning ready.

[koivunej]

Created spacejam/sled#1241

[koivunej]

The posted changes will remove this issue as flush_async() will no longer be used.