this is to make develop - debug cycle easiler when adding new features to service hub.

to enable this, turn on Roslyn/FeatureManager/Features/RemoteHost and RemoteHostInProc

once enabled, InProcRemoteHostClient will be used instead of ServiceHubRemoteHostClient

except concrete type of RemoteHostClient, all other parts, exactly same code will be used.

InProcRemoteHostClient can be used in unit test to test client and server side as they are.

This change also removes the vestiges of TestImpact out-of-proc build from
Roslyn (now that we can share an appBasePath without being in the same
extension folder, this service can move to its own package).

This change makes 'Workspace.DocumentActiveContextChanged' public and
introduces DocumentActiveContextChangedEventArgs to hold the relevant
set of arguments. All of its usages have also been updated.

This also introduces a breaking change to
RaiseDocumentActiveContextChangedEventAsync (a protected method of
Workspace).

added some comments and refactored some code.