Extend lifetime of client interceptor pipeline

[glbrntt]

Motivation:

A client call (i.e. the object the user holds) may live longer than the
transport associated with it (roughly speaking, the http/2 stream channel). An
example of this is when interceptors are use to retry and RPC and
redirect responses back to the original call.

However, the interceptor pipeline is held by the transport and is
currently set to nil when the transport is removed from the channel.
This means events invoked from the call object (such as cancellation)
which go via the transport (holding the interceptor pipeline) are
incorrectly failed.

Modifications:

• Have the client interceptor pipeline break the ref cycle between the
  transport and itself when the interceptor pipeline closes rather than
  when the transport is closed
• Emit a cancellation status rater than error on cancellation
• Update the ordering of when close is called in the interceptor
  pipeline.
• Add and update tests

Result:

"sub"-RPCs may be cancelled.

[Lukasa]

LGTM, nice change.