Make InspectorService supporting issuing most of its requests over the Flutter Daemon instead of observatory. (#1980)

* Make InspectorService supporting issuing most of its requests over the
flutter daemon instead of using the observatory protocol for requests
that do not need to use the observatory.

This avoids problematic cases where the observatory protocol allowed
requests to issue at surprising times such as right in the middle of
a flutter method call.
It is expected this fixes many hard or impossible to reproduce inspector
bugs.

To simplify the code I have also changed the semantics so that
the supportedServiceMethods set is computed before an InspectorService
is considered ready. This reduces the number of future.thenCompose
calls within the InspectorService code dealing with functionality only
exposed by the latest version.

* Address code review comments.

Author: jacob314