batch commit every 5 thousand changes, not 10 thousand

There's a tradeoff between making less frequent commits, and
needing to use memory to store all the changes that are coming
in. At 10 thousand, it needs 150 mb of memory. 5 thousand drops
that down to 90 mb or so.

This also turns out to have significant imact on total run time.
I benchmarked 10k changes taking 27 minutes. But two 5k batches
took only 21 minutes.

Assistant/Threads/Committer.hs

@@ -140,7 +140,7 @@ humanImperceptibleDelay = threadDelay $
 shouldCommit :: UTCTime -> [Change] -> Bool
 shouldCommit now changes
 	| len == 0 = False
-	| len > 10000 = True -- avoid bloating queue too much
+	| len > 5000 = True -- avoid bloating change pool too much
 	| length recentchanges < 10 = True
 	| otherwise = False -- batch activity
   where