* riece-channel.el: Require 'riece-cache.

* riece-channel.el: Require 'riece-cache.
(riece-find-channel): Increase priority of given channel name in
riece-channel-cache.
(riece-forget-channel): Remove given channel name from
riece-channel-cache.
(riece-get-channel): Register given channel name in
riece-channel-cache (if it is being added to riece-channel-obarray.)
* riece-server.el (riece-reset-process-buffer): Initialize
riece-channel-cache.
* riece-options.el (riece-channel-cache-max-size): New user
option.
* riece-misc.el (riece-get-channels-on-server): Get channel names
from riece-channel-cache.
* riece-globals.el (riece-channel-cache): New variable.