This commit doesn't change internal logic of any model, only the API to
be able to return result of connect TS asyncrhonously since some models
(like osmo-bts-trx) require some time to process the result. This way
PDCH ACT/DEACT (N)ACK can be sent once the result of this long process
is known. For instance, nowadays in osmo-bts-trx we PDCH (DE)ACT ACK
before getting the result from SETSLOT on the TRX iface.
With this new API, bts_model_ts_connect doesn't return any value
synchronously. Instead, it is expected to always end up calling
cb_ts_connected with the return code from the TS activation process. 0
is considered a successs, while any other value is considered an error.
Change-Id: Ie073a4397dd2f1a691968d12b15b8b42f1e1b0cf