Fix read from uninitialized memory bug in GrpcBufferWriter.

This commit fixes an issue in which the following sequence of operations
leads to use of uninitialized memory:

1. Caller invokes GrpcBufferWriter::Next(), and then makes use of 8191
   bytes in the returned buffer (which is 8192 bytes in size).

2. Caller then returns the unused single byte via
   GrpcBufferWriter::BackUp(). This method invokes
   g_core_codegen_interface->grpc_slice_split_tail(), which causes
   backup_slice_ to be a grpc_slice with one byte.

3. At the next invocation of GrpcBufferWriter::Next(), a reference to
   the single byte grpc_slice is returned to the caller.

The problem here is that the returned reference is to the inlined buffer
in the grpc_slice, which is resident in slice_, not the location of the
buffer inside slice_buffer_ after
g_core_codegen_interface->grpc_slice_buffer_add() in
GrpcBufferWriter::Next(). As a result, any data the caller writes to the
returned void* data is lost.

The solution is to avoid inlined backup slices.

Canonify server URI when setting the channel arg.

"/usr/bin/env python" in run_tests.py.

Stress test client service config (in kubernetes) need not be of 'LoadBalancers'.

Add Streaming ping-pong micro-benchmarks

Add Cronet core tests to Jenkins

Re-run yapf code, and pin version

Change shutdown_test.cc to use credential provider API to test against  different credential types

Automatedly advance ObjC version numbers

skip ruby tests on mac for time being

Update PROTOCOL-WEB.md

Mention the Server header format

Update PROTOCOL-WEB.md

1. Clarify about trailer-only responses
2. Switch to http/1.1 header block format for in-body trailer encoding
3. Clarify how compatibility is supported (using U-A and Server headers respectively)

PHP: update README and apache/nginx examples

Port #9505: Fixes for gRPC ObjC v1.1.0 release

Add a test verifying number of writes per RPC is reasonable

shorten qps worker timeouts and run remote workers under timeout

Add some debug utilities

Test CMake build on Linux and Windows continously

add missing return in scenario config