Merge release-0_15 into master

Network status tracking

Update C# examples to 0.15.0 packages.

Updated several threads to use CleanupThread for clean exit

Minor fixes

Remove caching of results by run_tests

Fix GPR_TIMER misbalancing

Basic testing of client and sever side compression for Python

Moved grpc_shutdown to end of Py_Finalize()

Fix clang formatting

Pass NULL in the host parameter of grpc_channel_create_call

Fix clang formatting

New epoll based polling implementation

fix building ruby package

Fix #3803

Re-enable accidentally disabled traces

Fix build error of SwiftSample project on Xcode 7.3 (Swift 2.2)

Modified route_guide sample app to show RPC log

Update version to 0.15.0

Update master branch to 0.16.0-dev

We currently rely on the __del__ method of a module scope object
to call grpc_shutdown().  __del__ methods are not guaranteed
to be called, and furthermore there are no guarantees about
ordering, leading to shutdown race conditions.  This moves
grpc_shutdown to Py_Finalize(), which gets called after the
Python context is completely cleaned up.

Allow Node users to set a custom logger and log verbosity.

Doc Fixit: Ordering and thread-safety for C++ streaming (sync/async) APIs