Move ServerSentEvents inside Redisse namespace
Coerce data to String when creating event
Use evented Redis for history events

* Switch from Redis to EM::Hiredis
* EM::Synchrony::Hiredis doesn't work with latest EM::Hiredis so the
  call to EM::Synchrony.sync is explicit.
* Had to use env.status instead of Goliath::API#status (actually
  method_missing) that uses Goliath::API#env which doesn't work with
  streaming responses.
Namespace Rack env key to "redisse."
More frozen strings
Move channels fetching out of #subscribe to #response
Use only one pubsub connection for all clients
Use Rack::Response to have a proper Content-Length