Implement Connection._close

This experimental feature will remove all resources used by the underlying libcouchbase instance.

Implementation details - in order to make sure a NULL pointer isn't being used for the various SDK APIs, a new "empty" object will be created - however it will not be initialized.

The reason for the underscore is:

1) This is not considered a normal feature. This is a rather internal aspect of the SDK and may cause the application to perform in unexpected (though not undetermined) state.
2) We don't want to mandate calling 'close()' as part of the API. For normal usage no specific 'close()' functionality is needed