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]

Change-Id: I952938474fa2780bf3c906cbdffb2d024b03c1b7