The new logger event object for making get_entry() block when no logs
are available was consuming the event's notification even if the thread
did not need to block. This was causing excessive blocking - if multiple
logs had been added since the last call to get_entry(), only one would
be returned, and the next call would block until yet another log was
added.
Now only call event::wait() to block the calling thread if there are no
logs available.
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