Handle signals properly when dropping GVL

src/ruby/ext/grpc/rb_call.c

src/ruby/ext/grpc/rb_completion_queue.c

src/ruby/ext/grpc/rb_completion_queue.h

src/ruby/ext/grpc/rb_grpc.c

src/ruby/ext/grpc/rb_server.c

src/ruby/ext/grpc/rb_signal.c

src/ruby/ext/grpc/rb_signal.h

src/ruby/lib/grpc.rb

src/ruby/lib/grpc/signals.rb