Logs that got sent sparsely or burstily would get buffered for long
periods of time since the logic to flush them only does so on the next
log write. In the worst case, a subsequent log write could never happen
and cause a log entry to be indefinitely buffered.
This fix introduces a recurring event/timer to simply flush all pending
logs at frequency of Broker::log_batch_interval.
This also installs symlinks from "zeek" and "bro-config" to a wrapper
script that prints a deprecation warning.
The btests pass, but this is still WIP. broctl renaming is still
missing.
#239
