PHP: persistent channel

src/php/ext/grpc/call.c

src/php/ext/grpc/call_credentials.c

src/php/ext/grpc/channel.c

src/php/ext/grpc/channel.h

src/php/ext/grpc/channel_credentials.c

src/php/ext/grpc/channel_credentials.h

src/php/ext/grpc/php7_wrapper.h

src/php/ext/grpc/php_grpc.c

src/php/ext/grpc/php_grpc.h

src/php/tests/unit_tests/CallTest.php

src/php/tests/unit_tests/ChannelTest.php

src/php/tests/unit_tests/EndToEndTest.php

src/php/tests/unit_tests/SecureEndToEndTest.php