Persist Blowfish keys in the database

Add a new text field called "cipher" and store the hex-encoded
cipher key there whenever a new key is set or exchanged.  Also,
when each network is initialized, load the ciphers out of the
database and initialize the in-memory hashmap.  Then, the existing
behavior of each CoreIrcNetwork automatically using these keys
upon construction occurs.

Additionally, this makes PM buffer ciphers persistent both across
destruction/construction and across core restarts.

Note that the existing "key" field in the database is confusingly
named.  It does not contain any sort of cryptographic key but
instead holds channel passwords.

Closes #1473

Closes GH-332.