Made PHP library use only a single completion queue

src/php/ext/grpc/call.c

src/php/ext/grpc/call.h

src/php/ext/grpc/completion_queue.c

src/php/ext/grpc/completion_queue.h

src/php/ext/grpc/config.m4

src/php/ext/grpc/php_grpc.c

src/php/ext/grpc/server.c

src/php/ext/grpc/server.h