GH-780: Prevent log batches from indefinite buffering

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.

testing/btest/Baseline/broker.remote_log_batch/recv.test.log

@@ -0,0 +1,11 @@
+#separator \x09
+#set_separator	,
+#empty_field	(empty)
+#unset_field	-
+#path	test
+#open	2020-02-05-20-53-15
+#fields	msg	num
+#types	string	count
+ping	0
+ping	1
+#close	2020-02-05-20-53-16

testing/btest/Baseline/broker.remote_log_batch/send.send.out

@@ -0,0 +1 @@
+Broker::peer_added, 127.0.0.1

testing/btest/Baseline/broker.remote_log_batch/send.test.log

@@ -0,0 +1,11 @@
+#separator \x09
+#set_separator	,
+#empty_field	(empty)
+#unset_field	-
+#path	test
+#open	2020-02-05-20-53-15
+#fields	msg	num
+#types	string	count
+ping	0
+ping	1
+#close	2020-02-05-20-53-16