Fix milliseconds in certain cache.log messages (#1167)

Fix milliseconds in certain cache.log messages (#1167)

This change fixes two bugs:

1. The milliseconds part of an early message time was wrong because
   debugLogTime() got it from the current_time global instead of the
   saved message timestamp. This problem probably affected only early
   saved messages in rare situations where current_time was updated
   between message creation and message writing. This fix adjusts
   DebugMessageHeader::timestamp to store message time with microsecond
   precision.

2. Level-0/1 messages that followed same-second level-2+ messages were
   logged with an unwanted (and stale) milliseconds value. This fix
   forces a reset of the message line format buffer in relevant cases.