Add POP3 support

[rstml]

Current master already includes james-protocols based LMTP implementation and POP3 should be quite easy to add.

Some design notes:

• We need to add new reserved pop3 system label for POP3 maildrop. Messages which belong to this label will be available to POP3 clients.
• Client can only fetch limited, preconfigured number of messages (e.g. 500).
• For POP3 UIDL we will use Base64 encoded UUID. According to RFC1939 UIDL has 70 octets limit and can be in a range between 0x21 to 0x7E.
• DELE command will delete only from pop3 label (in future may be configured to delete message from inbox label as well)

[rstml]

This feature is blocked by https://issues.apache.org/jira/browse/PROTOCOLS-98