avoid potential Logger deadlock in SIGQUIT and SIGUSR1

avoid potential Logger deadlock in SIGQUIT and SIGUSR1

If any combination of SIGQUIT and SIGUSR1 are sent to a
Rainbows! worker in a /very/ short period of time, the Mutex
used by the default Logger implementation may deadlock since
Mutex synchronization is not reentrant-safe.

Users of alternative logger implementations (or monkey-patched
ones) are possibly not affected.  Users of the logger_mp_safe.rb
monkey-patch distributed[1] with unicorn are not affected.

[1] http://unicorn.bogomips.org/examples/logger_mp_safe.rb