core: prepend seconds.microseconds since epoch to logging messages

refs: #1440

Change-Id: I4ed478e574cf4eff68c3e80b3db1a6dbaab54089
core: change NFD_LOG_* macros to use std::clog

This change makes all logging output BUFFERED

refs: #1440

Change-Id: Ief60c43b532e4524a70aad2bf15aa06490847535