This implements the following main pieces of functionality:
* Renames key spec "CHANNEL" to be "NOT_KEY", and update the documentation to
indicate it's for cluster routing and not for any other key related purpose.
* Add the getchannels-api, so that modules can now define commands that are subject to
ACL channel permission checks.
* Add 4 new flags that describe how a module interacts with a command (SUBSCRIBE, PUBLISH,
UNSUBSCRIBE, and PATTERN). They are all technically composable, however not sure how a
command could both subscribe and unsubscribe from a command at once, but didn't see
a reason to add explicit validation there.
* Add two new module apis RM_ChannelAtPosWithFlags and RM_IsChannelsPositionRequest to
duplicate the functionality provided by the keys position APIs.
* The RM_ACLCheckChannelPermissions (only released in 7.0 RC1) was changed to take flags
rather than a boolean literal.
* The RM_ACLCheckKeyPermissions (only released in 7.0 RC1) was changed to take flags
corresponding to keyspecs instead of custom permission flags. These keyspec flags mimic
the flags for ACLCheckChannelPermissions.
