core: make log statements more clear

Improve log statements in DbUserCacheManager and
UsersDomainsCacheManagerService - remove "new Date", improve the log
statement itself, add logging statements for start/finish initialization

Change-Id: I87111a8c5ca3392dca77944252687fdca9d3588d
Signed-off-by: Alissa Bonas <abonas@redhat.com>