switch to Celluloid and Sequel

This is sort of the bare minimum conversion, it doesn't really take
proper advantage of Celluloid yet. There's probably stuff I've missed
too.

The switch to Sequel was required because otherwise I'd have to wrap
every single ActiveRecord call in
ActiveRecord::Base.with_connection { ... } in order to avoid leaking
connections and eventually maxing out the connection pool. Sequel
immediately returns the connection to the pool when it's done with the
transaction, rather than holding on to it until the thread explicitly
releases it.
basic SSL listener support
WIP new supervisor
use new celluloid ssl accept support
s/bang/async
replay backlog in a disk threadpool

also fix a bug with replaying private messages
replay the topic on client join

closes gh#25
fix backlog bug with strftime

fixes gh#24
AR -> Sequel query conversion
protect against deleted network