Refactoring of SPI

This is a preliminary pull request to simplify the SPI. The goal is to no longer require connectors to have to implement canHandle(...) and test for connectorId, which is repetitive and error prone.

I haven't yet removed the HandleResolvers due to a couple of bugs in Jackson:
https://github.com/FasterXML/jackson-databind/issues/406
https://github.com/FasterXML/jackson-databind/issues/408