res_stasis: Fix crash on a hanging up channel.

* Give the struct stasis_app_control ao2 object a ref to the channel held
in the object.  Now the channel will still be around if a thread needs to
post a stasis message instead of crash because the topic was destroyed.

* Moved stopping any lingering silence generator out of the struct
stasis_app_control destructor and made it a part of exiting the Stasis
application.  Who knows which thread the destructor will be called under
so it cannot affect the channel's silence generator.  Not only was the
channel unprotected when the silence generator was stopped, stasis may no
longer even control the channel.

ASTERISK-25882

Change-Id: I21728161b5fe638cef7976fa36a605043a7497e4