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.
SERVER-5553 Eliminate malloc from printStacktrace(std::ostream&).

This also opens the exciting possibility of displaying the base address
of the file containing the address, which would let us dump stacks when
ASLR is in use.  However, this implementation preserves the format used
in the Linux backtrace_symbols() implementation.