Changes

stream_cli: Support destroy object within user callback

This commit improves the code in stream_cli to allow the user calling
osmo_stream_cli_destroy() while in a callback.

For instance, the following scenario:
read_cb(len=0)
 osmo_stream_cli_reconnect()
  osmo_stream_cli_close()
   disconnect_cb
    [user calls osmo_stream_cli_destroy()]
 [popped stack does read_cb()]
   [user uses freed msg]

This allows using stream_cli by users handling destruction and reconnect
of stream_ci objects on their own, such as libosmo-abis
e1_input/sign_link stack.

Change-Id: I952938474fa2780bf3c906cbdffb2d024b03c1b7