Enable Infinispan SPIs to implement their own commands

Much nicer IMHO would be a nice SPI / plug point to allow applications to extend the set of commands. Let an app create an ExtensionCommand; if that comes in off the channel pass it off to the handler the application registered.

This could for example be used instead of the solution currently used in http://opensource.atlassian.com/projects/hibernate/browse/HHH-3818. In HHH-3818, the cache is used as a notification vehicle which is a bit of a hack and has some limitations for example when trying to implement a similar solution for evict(key) op.

Another use case for it would be HTTP session ownership, from Brian Stansberry:

(09:32:09 AM) brian_stansberry: ... it's just that only 1 server at a time s/b able to use a session
(09:32:25 AM) brian_stansberry: so on failover i send an extension command requesting ownership
(09:33:19 AM) brian_stansberry: there's actually a servlet spec requirement that only one server at a time access a session
(09:33:23 AM) brian_stansberry: ejb spec too
(09:33:37 AM) brian_stansberry: we don't properly handle that
...
(09:34:18 AM) brian_stansberry: basically it's a lock
...
(09:34:35 AM) brian_stansberry: that's all it is, a distributed lock
(09:35:03 AM) brian_stansberry: but once you have the distributed lock it's yours until you give it up
(09:35:29 AM) brian_stansberry: so once 1 request acquires it, subsequent requests to same server can just use a local lock
...
(09:41:59 AM) brian_stansberry: the key thing is there should only be a cluster-wide operation once per session
(09:42:11 AM) brian_stansberry: unless there is failover