eventlog: Fix various races

Previously the eventlog infrastructure had a couple of races that could
pop up when using the startEventLog/endEventLog interfaces. In
particular, stopping and then later restarting logging could result in
data preceding the eventlog header, breaking the integrity of the
stream.

To fix this we rework the invariants regarding the eventlog and
generally tighten up the concurrency control surrounding starting and
stopping of logging.

We also fix an unrelated bug, wherein log events from disabled
capabilities could end up never flushed.