Update dovecot2 to 2.2.15, and add an LZ4 option that uses liblz4.

ChangeLog:

Some small fixes and changes to v2.2.14. This release is mainly in the hope
that it could still make it into the next Debian stable instead of v2.2.14 -
mainly because of a couple of new assert crashes that started happening in
v2.2.14 and should be fixed now.

• Plugins can now print a banner comment in doveconf output

	  (typically the plugin version)

• Replication plugin now triggers low (instead of high) priority for

	  mail copying operations.

• IMAP/POP3/ManageSieve proxy: If destination server can't be

	  connected to, retry connecting once per second up to the value of
	  proxy_timeout. This allows quick restarts/upgrades on the backend
	  server without returning login failures.

• Internal passdb lookups (e.g. done by lmtp/doveadm proxy) wasn't

	  returning failure in some situations where it should have (e.g.
	  allow_nets mismatch)

• LMTP uses mail_log_prefix now for logging mail deliveries instead of

	  a hardcoded prefix. The non-delivery log prefix is still hardcoded
	  though.

+ passdb allow_nets=local matches lookups that don't contain an IP

	  address (internally done by Dovecot services)

+ Various debug logging and error logging improvements

• Various race condition fixes to LAYOUT=index
• v2.2.14 virtual plugin crashed in some situations