SimpleLogger is not fully thread-safe, and all loggers that wrap it are
also not safe accordingly. this does not affect much, but in rare cases
it can cause interleaving of messages on stderr when used with the json
or raw log formats. the fix applied here is a bit of a hack, but fixing
this properly requires rearchitecting the logger infrastructure. nested
loggers are not the most natural abstraction here, and it is biting us.
Change-Id: Ifbf34fe1e85c60e73b59faee50e7411c7b5e7c12
0109368c3f