This is in preparation of moving all tests into a separate package to
enable ease of coverage checking and testing.

Fixes #1591.

Also fixes a raised-exceptions-during-tests-causes-hanging problem.

Also adds the commands to the grpc_docker shell library script to
support running in docker.

Refactors early_adopter such that interface-specific code is located in
the framework module as a submodule `alpha`.

Also refactor the interop package to perform client-server interop
testing in a unit test.

The server_host_override flag's implementation remaining
outstanding means that this only works over insecure
connections for now.

This makes grpc.early_adopter much more independent of RPC
Framework and cleaner at the cost of reexporting most of the
interfaces and writing several delegation classes.

A top-level "grpc" package is added and RPC Framework
is made public.