[CHA-RC3] Channel options / modes overwritten

[sacOO7]

At the moment, channel options / modes on shared channels are determined by LWW. So if you have all the features, the options provided by messages override those provided by occupancy.

Shared channels need to be configured such that they merge the modes required by each feature and request the channel only once.

Same as -> ably/ably-chat-js#411
Related impl -> ably/ably-chat-js#415

┆Issue is synchronized with this Jira Task by Unito

[sacOO7]

This needs a separate specification point as per ably/ably-chat-js#411 (comment)

[sacOO7]

Specification already defined as per CHA-RC3