Python stress test docker config files and related changes

Manual upmerge from 0.14.0.

Add necessary build-from-source command to documentation

Adds a command that allows the instructions to be used to install gRPC Python
from scratch.

Conflicts:
	Makefile
	build.yaml
	package.json
	src/core/lib/surface/version.c
	src/csharp/Grpc.Core/VersionInfo.cs
	src/csharp/build_packages.bat
	src/node/tools/package.json
	src/python/grpcio/grpc_version.py
	src/ruby/lib/grpc/version.rb
	src/ruby/tools/version.rb
	tools/distrib/python/grpcio_tools/grpc_version.py
	tools/doxygen/Doxyfile.c++
	tools/doxygen/Doxyfile.c++.internal
	tools/doxygen/Doxyfile.core
	tools/doxygen/Doxyfile.core.internal

Master is now 0.15.0-dev.

Ruby fixes - for the release branch this time...

The release branch is now 0.14.0-pre1.

Continuation of #6280.

debug logging fix

Fix C# distrib tests after getting rid of grpc.native.csharp package

Modified sample async client to be multi-threaded  

Temporary fix for plugin initialization problem

C# improve handling of serverside calls with unary response.

cronet wrapper code

Ruby: improve server error handling, fix a reference error

Improve docs on GRPC_OP_RECV_CLOSE_ON_SERVER

add a dummy service with no rpc to test codegen coverage

Add proto comments to generated _pb2.py files

Removed leftover function declaration from channel args

Fixed name of test config for h2_census

Fix coverage for large message and secure async end2end

Don't try to read files that have been moved/renamed.

Moved protos out of core_codegen interface