logd: wakeup on wrap or timeout

If a timeout is specified for the reader, then go to sleep
with the socket open. If the start time is about to get
pruned in the specified log buffers, then wakeup and dump
the logs; or wakeup on timeout, whichever comes first.

Bug: 25929746
Change-Id: I7d2421c2c5083b33747b84f74d9a560d3ba645df