Using the same namespace results in errors when using the c-extension
version of protobuf due to name collisions

This brings gRPC Python into conformance with the recommendation in
https://github.com/grpc/grpc/blob/master/doc/PROTOCOL-HTTP2.md.

A weak test is included; proper test coverage will come later.

... but for real this time.

In those places where we return an object that implements two
interfaces (let's say P and Q), consistently describe it as a "P-Q"
rather than once mentioning that it is a P and describing it as a "Q"
for the rest of prose.

The test now expects to receive StatusCode.INTERNAL when it receives a
RST_STREAM from the server in order to comply with
https://github.com/grpc/grpc/blob/master/doc/PROTOCOL-HTTP2.md

Also added wait for ready behavior on the client channel.

This should have been included in dd52a313
but was missed because it falsely passes almost all of the time.

This reads directly off of the slices rather than ref'ing and unref'ing
them. There's likely some silliness w.r.t. interned slices and
references to them outliving their associated call objects, but we are
just ignoring that for now.

The clients now block until the channel is in the READY state. This
fixes some test flakiness issues we have had.

It's been confusing that these tests have been called "interop" but
are not actually tests of interoperation.

Added tests for detecting invocation time defects arising out of the
runtime.

Cast iterables into iterators for stream based compression, empty
message, and metadata tests.

Restructure the consume_request_iterator method to handle exceptions
where the object being passed is not an iterator. Fixes #8231 and #8454.

Documentation for the new tests can be found https://github.com/grpc/grpc/blob/master/doc/negative-http2-interop-test-descriptions.md

Add a comma delimiter for "_invalid_metadata_test.InvalidMetadataTest".

Apparently Exception.message was introduced in Python 2.5 and
deprecated almost immediately afterward in Python 2.6.