rusty-gd: reorg hci internals

with the intent of adding dynamically registered event receivers:

1. combine incoming and outgoing into a single selected dispatch (to
eliminate need to lock on pending commands)

2. add dynamically registered event receivers

Bug: 171749953
Tag: #gd-refactor
Test: gd/cert/run --rhost
Change-Id: I038364bc8d99b7ead4688f7570b79826fa7a1651