Contains squashed commits for several typo fixes and comments made during review.

As part of this, get the deadline from a channel arg instead of pick_args.

Added a missing ref in httpcli.c. Also added a test to avoid
regression

This changes the import style for proto dependencies from a) to b).
a) "import a.b.c as x"
b) "from a.b import c as x"

Using statement a) causes problems when using __init__ files. If module
"a.b" has an __init__.py file which is importing the python generated
grpc code "a.b.d", then we cannot import a module named "a.b.c" because
the module "a.b" does not exist yet. In this case python will throw:
AttributeError: 'module' object has no attribute 'b'

This PR adapts the code to use the same logic as in:
/google/protobuf/compiler/python/python_generator.cc

See http://httpwg.org/specs/rfc7540.html#BadCipherSuites

This reverts commit 79759fea, reversing
changes made to dc36f4df.

- using NULL-terminated strings as opposed to const unsigned char *
with length since the credentials are in PEM format.
- aligning the structures with gRPC so that we have less convertions to
do back and forth.

Renamed remove and add static functions
to remove_key and add_key