binary format for persistent history messages

[slingamn]

A very large proportion of the size of a persistent history message is JSON bloat. As described here:

https://github.com/oragono/oragono/blob/cb356ecaf01b87daf1ed67edf5ea8f30bce02f7f/irc/mysql/serialization.go#L10

it is easy to add a binary format with a different magic number and then support retrieval of both formats.