Move C-layer call creation into GRPCChannel, so that it always acts on a
non-destroyed channel.

Listen for connectivity in the GRPCCall, and destroy the channel.

- Revised 'protoc' section and 'Documentation' section as LisaFC suggested.
- Added 'Examples' section as LisaFC suggested.

Resolves: #5661

Obvious in hindsight, if the cas failed, we still created the subchannel
call object and then left it dangling.

Fixes a leak observed on master in the past 48 hours.

It was possible for entries in watchers[] to be deleted by another thread before we called begin_poll.

I'd like to do something nicer than this eventually... but that'll be easier once we've got the polling loop cleaned up (which is currently WIP)

Revert "Pass a non-infinite deadline to grpc_completion_queue_next() to prevent queues from blocking indefinitely in poll()"