Summary:
Thread-pool pops a thread function and then run the function,
which may cause thread-pool is empty but the last function is still
running.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/9502
Test Plan:
`gtest-parallel ./env_test
--gtest_filter=DefaultEnvWithoutDirectIO/EnvPosixTestWithParam.RunMany/0
-r 10000 -w 1000`
Reviewed By: ajkr
Differential Revision: D34011184
Pulled By: jay-zhuang
fbshipit-source-id: 8c38bef155205bef96fd1c988dcc643a6b2ac270