liblog: resolve deadlocks

Although ever present, an increased regression introduced with
commit b6bee331 (liblog: logd:
support logd.timestamp = monotonic).

A signal handler can interrupt in locked context, if log is written
in the signal handler, we are in deadlock. Block signals while we
are locked. Separate out timestamp lock from is loggable lock to
reduce contention situations. Provide a best-guess response if
lock would fail in timestamp path.

Bug: 25563384
Change-Id: I6dccd6b99ebace1c473c03a785a35c63ed5c6a8a