gRPC.podspec shouldn't be a template anymore.

Upmerge 0.14 -> 0.15

Conflicts:
	gRPC.podspec
	src/objective-c/BoringSSL.podspec
	src/objective-c/examples/RemoteTestClient/RemoteTest.podspec
	src/objective-c/examples/Sample/Sample.xcodeproj/project.pbxproj
	src/objective-c/examples/SwiftSample/SwiftSample.xcodeproj/project.pbxproj
	src/objective-c/tests/Podfile
	templates/gRPC.podspec.template
	third_party/protobuf

Add arm_arch.h back in BoringSSL.podspec, to fix compilation for devices

Fix _Rendezvous.exception for successful calls

Port server change 0.14

Change port_server.py to use port 32766

Bumped grpc-tools version

Use open-source defaults to propagate Python plugin configuration

32767 is used by filenet-powsrm

PHP: fix shutdown hang issue

Fix Cocoapods linter issues with gRPC-Core.podspec

Make handlers optional at server construction

Fix Cocoapods linter issues with BoringSSL.podspec

This works around the Cocoapods' linter header_mapping_dir restrictions.
Incidentally, it also removes the need to set build settings in the user
target.

Restore fix undone by https://github.com/grpc/grpc/pull/5893

That PR wasn’t tested nor reviewed adequately.

Small C# docs improvements