Implement grpclb drop support.

Upmerge v1.3.x branch to master

C#: Dont use non-blittable types in native method signatures

Fix runtests --force_default_poller on linux

Unref pending events under cq lock

Doxygen cpp nits

PHP: fix pecl extension version

PHP: fix pecl extension after cc files are added

Add Backward compatibility API to C++

Fixes to subchannel and its index

Fix hpack_size test

PHP: stop requiring google/protobuf PHP implementation

Try fixing python artifact build on Windows

Fix run_tests script for Mac

Switch default CQ count for thread manager to num_cpus

Fix some job runner bugs, save some initialization passes for port selection

Bump to version 1.3.3

Remove OSS-only tooling