remove channel upon undeclaration of Subscriber and Queryable

[Charles-Schleich]

Fixes #139

Note: Due to the timing difference in between issuing an UndeclareSubscriber from the front-end to the plugin, this change may cause the TS bindings to warn that a "Subscrption UUID not in map" Depending on the number of outstanding messages that still need to be flushed from the plugin before the undeclare is executed in the plugin.

The state of a Subscriber lives on the Plugin, we have changed the way the TS client and the plugin handle undeclaring Subscribers.
The flow is as follows:

1. TS client sends Undeclare Message to Plugin with Subscruber UUID
2. Plugin aborts Future that Subscriber lives on (effectively undeclaring the Subscriber as per Drop impl in Zenoh), it is gaurenteed no more messages will be sent to the client once this future has been aborted.
3. Plugin explicitly sends back to the client an undeclare message with the SubscriberUUID to the TS client, which then removes the SubscriberChannel from the corresponding map in the RemoteSession.

This should resolve the uncaught exception that occurs when another node floods this client with many samples, and allow a new subscriber to be declared on the same session.

[github-actions]

PR missing one of the required labels: {'bug', 'breaking-change', 'documentation', 'enhancement', 'new feature', 'dependencies', 'internal'}

[milyin]

Fixes #139

Note: Due to the timing difference in between issuing an UndeclareSubscriber from the front-end to the plugin, this change may cause the TS bindings to warn that a "Subscrption UUID not in map" Depending on the number of outstanding messages that still need to be flushed from the plugin before the undeclare is executed in the plugin.

Not clear how this solution can provoke "Subscrption UUID not in map" warning. As far as I see nothing happens until the moment when "Undeclare" reply is received by zenoh-ts from plugin

[Charles-Schleich]

@milyin I have updated the comment on the Issue, to clarify the new behaviour.