Async hangs up twice

[Jiri Svoboda]

For sessions of type EXCHANGE_ATOMIC or EXCHANGE_SERIALIZE async_hangup() will call async_hangup_internal() twice on the same phone.

This is because in the first call async_exchange_begin() an exchange is created with phone equal to the session phone. In async_hangup() we first hang up the session phone, then all the phones of inactive exchanges. In this case there will be one inactive exchange with phone equal to the session phone.