Sometimes a client can get confused, and start sending messages with
null client ids. This just avoids crashing when that happens.

You need to provide callbacks to redis commands or else redis will throw
an error (killing your process) when something goes wrong.

When a client is subscribed to channels like /foo/* and /foo/bar, and a
message is published to /foo/bar, the client should only receive *one*
message. Previously, we were sending the message for every shard each
subscribed channel lived in. (So if /foo/* was in shard1 and /foo/bar
was in shard2, the subscriber would get two copies.)

This change collects all the client ids from all shards and only sends
one message to each client.

…timeout

Pulled in "async" to help with several cases where we perform several
operations in parallel, and need a callback at the end.

Ported from faye-redis-node, see:

https://github.com/jpignata/faye-redis-node/commit/d0bc94086315ddbffbcf68018df4e0019eb3c632