Provide a way to pause/resume inbound and outbound traffic (connector-based)

[cescoffier]

This issue is about providing a way to pause and resume inbound and outbound traffic.
Basically, it would introduce Pauseable inbound channels and outbound channels which once paused:

• inbound: will stop the poll/querying, but accumulating the downstream requests but not passing them to the "client"
• outbound: will stop the write of messages, but accumulating the message and stop making new requests

These pause/resume methods can be called on each individual channel if they are handled by compatible connectors. It will also be possible to call a global "pause" which will suspend all compatible channels and allow emitters to check that state (and so stop emitting).