Replace the existing user agent string in SetUserAgentPrefix

Introduce grpc_completion_queue_factory API

Prevent cleaning up the fd twice.

Use 7-bit prefix in manually created header.

fix the namespacing for php docs on client classes

Fix tcp_server_utils_posix_common

Upmerge from 1.2.0

Remove spaces for correct link format in README.md.

Fix markdown

Add socket factory support to udp_server.c

Fix ruby mingw build, add missing header

Python Service Reflection

bump v1.2.x branch to 1.2.1-pre1

add http2 testing interop server uses small data frames and padding

ruby add background channel state poller to handle disconnected transports

There is a behavioral discrepency in
`DescriptorPool.FindFileContainingSymbol` between the two
implementations of the protobuf python library (one of which is purely
python, and the other utilizes a python extension library written in
C++).

protobuf on PyPI is distributed via various prebuilt wheels, and our CI
setup fetches the relevant one for each platform.  Our Linux instances
end up fetching a pre-compiled C++ version of the library, whereas our
Mac and Windows builders automatically get the pure python version.
This test fails (and our reflection implementation itself) succeeds with
the C++ implementation but fails with the pure python version.

The test will be temporarily disabled pending a fix for
https://github.com/google/protobuf/issues/2882

While not strictly required by the protocol,
it will make testing easier.

Fix and un-skip the test for _file_containing_extension
in python reflection service to refer to the correct proto
and expect the right object to be returned. Enable it too.

Drop unnecessary framework imports from generated _pb2_grpc.py files.

Error slice optimization

Expand benchmark coverage to isolated grpc_call scenarios against a dummy filter stack