Convert buserver to libutil's logging

The primary motivation is to get automatic log rotation, but we
also get built-in locking for the logging calls.

This does have a side effect of changing the format used to print
timestamps in the log file.

Garbage-collect the unused (and un-exported) printHashTable().

Change-Id: I860370e60082ea355806b3f1945eee76db7c32e3