Increase gerrit sendemail thread pool size

We're seeing a growing backlog of email events that all must funnel
through the single thread allocated to sending email. We think this may
be related to server slowness that we are observing. Bump the number of
threads to 4 to see if that flushes the queues quicker and gives us a
more responsive server.

Change-Id: I730c8f657191cedb46f81f4abc0e1796ef847b27

playbooks/roles/gerrit/templates/gerrit.config

@@ -20,6 +20,7 @@
 	smtpServer = localhost
 	from = MIXED
 	includeDiff = false
+	threadPoolSize = 4
 [container]
 	user = gerrit2
 	startupTimeout = 300