logd: correct duplicate message state machine

Inspection turned up that for the case of three identical messages,
the result would be a stutter of the first message only.  Added
comments to describe the state machine, incoming variables, outcoming
and false condition outputs, for proper maintenance in the future.

Test: gTest liblog-benchmarks BM_log_maximum* and manually check
      for correct midstream chatty messages,
Bug: 33535908
Change-Id: I852260d18a484e6207b80063159f1a74eaa83b55