Add a maybe-temporary way for apps to clear the channel cache

fix for working with compressed data. Fixes #6746.

Use open-source defaults to propagate Python plugin configuration

Fix bug with override_server flag in run_interop_tests.py

Dont create errors redundently in chttp2_transport.c

implemented eventfd_check_availability

Fix Python server interop spec noncompliance

PHP: fix shutdown hang issue

Updated git clone URL

fixed size_t format string

Changed default string type to be str

Build experimental nugets with dotnet CLI

Make transport-level errors be reflected in status messages on calls

Re-enable two Node credentials tests

This impacts the following APIs:
  Metadata: Key is always a str, Value is bytes for binary metadata,
    str otherwise
  Call Details: str type
  gRPC method: str type
  hostname/target: str type

Also fix spelling mistakes in the spec itself.

Convert windows dll filename to bytes

Lowered six dependency version to 1.5.2

Build ObjC examples in travis

Removed _connectivity_channel tests

Trace statements in epoll poller (enabled by default to debug the recent perf regression)

Fix InteropTests.m

Give existing port_server time to respond