Fix invalid memory access in root canal.

The aysnc sockets depend on the async manager. We now make sure we do
not delete the async_manager_ before all the sockets have been
deleted.

This caused crashes and hangs in the emulator.

Test: gd/cert/run
Bug: 197160153
Change-Id: I692ed0d183c9dec84a6280de0450c1f3f74b0b13