Review and backport EOS PR 3037 log appender mutex

[abitmore]

https://github.com/EOSIO/eos/pull/3037

[pmconrad]

Careful, a mutex in a logger is dangerous for performance.

[jmjatlanta]

This is why stderr and stdout are horrible for logging in multi-threaded apps. My advice is to add the mutex but document the performance penalty to discourage its use.