Make a string parser interface for client endpoints that takes the reactor as a parameter in its signature

[ashfall]

At present, IStreamClientEndpointStringParser.parseStreamClient doesn't take the reactor as a parameter, which is bad. As a result, even though endpoints.clientFromString needs a reactor to be passed as a parameter, it cannot pass it on to string description plugins for client endpoints, and we end up having to use a global reactor in the plugin, which is bad too.

We need a new interface with the correct signature defined, a new plugin interface to go with it, the plugin loader updated to load that new kind of plugin and call its method correctly, deprecation warnings for use of the old interface, and the existing plugins/parser in Twisted to be ported to the new interface.

Can/should probably be split into smaller tickets.