Fixed math and stock servers

Adding interop client for C#

Updated event.cc with new changes

New Core API

Move completion_queue.cc to common/ from server/

Clean build.json

Autohinting for write buffering

Polishing C# math service implementation and adding inprocess tests

Few minor fixes.

Making the gpr_tmpfile's win32 version a bit more Windows-y.

Fixing a few issues in case you *cough* have a different local implementation of
protobuf, gtest and gflags for some weird reason.

V0 implementation of census_get_active_ops().

Use environment variable to set root certs in c++ interop test

Adding /tmp to prefix when creating posix temporary files.

- Addresses #416.

Also adding windows helpers to convert to and from TCHAR strings.

Improved C# grpc server implementation to be able to register call handlers