increase dovecot imap_idle_notify_interval to 4 minutes

Doesn't seem like 2 minutes is a problem, but 4 minutes seems better. A little less bandwidth, possibly less battery usage (though we don't have evidence that's actually true), and the interval should be shorter than any peer timeouts that might occur due to inactivity

fixes #129

setup/mail-dovecot.sh

@@ -53,6 +53,15 @@ tools/editconf.py /etc/dovecot/conf.d/10-ssl.conf \
 sed -i "s/#port = 143/port = 0/" /etc/dovecot/conf.d/10-master.conf
 sed -i "s/#port = 110/port = 0/" /etc/dovecot/conf.d/10-master.conf
 
+# Make IMAP IDLE slightly more efficient. By default, Dovecot says "still here"
+# every two minutes. With K-9 mail, the bandwidth and battery usage due to
+# this are minimal. But for good measure, let's go to 4 minutes to halve the
+# bandwidth and number of times the device's networking might be woken up.
+# The risk is that if the connection is silent for too long it might be reset
+# by a peer. See #129 and http://razor.occams.info/blog/2014/08/09/how-bad-is-imap-idle/.
+tools/editconf.py /etc/dovecot/conf.d/20-imap.conf \
+	imap_idle_notify_interval="4 mins"
+
 # LDA (LMTP)
 
 # Enable Dovecot's LDA service with the LMTP protocol. It will listen