SERVER-5553 Remove a heap allocation from the log event encoder.

The goal is to eventually have a logging path that is truly safe to
use in a signal handler.  This patch advances the cause by removing
the heap allocation caused by returning a string in the date formatters.

Conflicts:
	src/mongo/db/server_options_helpers.cpp
	src/mongo/logger/message_event_utf8_encoder.cpp
	src/mongo/util/time_support_test.cpp