Partial support for AMQP heartbeats

Provide support for AMQP heartbeats in `wait_frame_inner()`. Heartbeats will
be sent/recv'd when this function is called and it enters a
long-blocking wait.

From an library-user's perspective heartbeats are only handled inside of
`amqp_simple_wait_frame()` (and any RPC function). They are not handled in
`amqp_basic_publish()` nor anything outside the library.