logd: enhance multiple blocking readers performance

logd suffers performance degradation when multiple blocking
readers connect to it. Each time when the writer thread log
new entries, all of the readers are notified regardless of
which log id they are watching.

In this case, only give notification to the readers who are
actually watching new entries' log id. This decreases logd
CPU consumption by skipping unnecessary LogBuffer::flushTo
loops.

Test: liblog-unit-tests, logd-unit-tests & CtsLiblogTestCases
      logcat-unit-tests
Test: manual:
1.'logcat –b all' at constant heavy logging load level
2.simultaneously 'logcat –b crash' in another session,
   a healthy crash buffer usually keep empty
3.logd CPU consumption doesn't increase after step 2

Change-Id: I4ffc045c9feb7a0998f7e47ae2173f8f6aa28e8a