Allow the Deferreds returned by ClientCreator's methods to be cancelled

[exarkun]

Cancelling the Deferred returned by ClientCreator's connectTCP, connectSSL, and connectUNIX methods should fail the Deferred with CancelledError and abort the connection attempt by calling stopConnecting on the IConnector provider returned by the underlying reactor method.

[exarkun]

(In [28581]) Branching to 'clientcreator-cancellation-4329'

[exarkun]

Done, and with only about 40% more insanity than the RDA.

[therve]

• It would be nice to remove some duplications in ClientCreator, maybe making the canceller a method? Although it may be hard because it relies on the local frame.
• test_cancelConnectSSLTimeout mentions connectUNIX in its docstring.

It's going to conflict pretty badly with #1442, because of the changes in proto_helpers, so it would be nice to notify David. Other than that, it looks good, +1!

[therve]

Oh, and the branch misses a NEWS fragment.

[exarkun]

(In [28623]) Refactor ClientCreator methods to use a common implementation

refs #4329

[exarkun]

(In [28624]) Correct docstring

refs #4329

[exarkun]

(In [28631]) Merge clientcreator-cancellation-4329

Author: exarkun Reviewer: therve Fixes: #4329

Implement cancellation for the Deferreds returned by ClientCreator's methods.