* metadata spec update: disallow colon entirely
* refactor key validation
* implement metadata before-connect
* play the metadata in reg burst to all clients with the cap
* bump irctest
* remove all case normalization for keys
From spec discussion, we will most likely either require keys to be lowercase,
or else treat them as case-opaque, similar to message tag keys.
* refactoring
* send an empty batch if necessary, as per spec
* don't broadcast no-op updates
* don't trim spaces before validating the key
* bump irctest to cover metadata
* replay existing metadata to reattaching always-on clients
* use canonicalized name everywhere
* use utils.SafeErrorParam in FAIL lines
* validate key names for sub
* fix error for METADATA CLEAR
* max-keys is enforced for channels as well
* remove unlimited configurations
* maintain the limit exactly without off-by-one cases
* add final channel registration check
