shared/gatt-client: Speed up discovery

shared/gatt-client: Speed up discovery

This tune the discovery to take advantage of the cached database whenever
possible, so instead of clearing the whole db if the device is not paired
the code now make the remote services active once they are discovered
and with that bt_gatt_client can then skip discovering characteristics
and descriptors of services that have not changed since last connection
which improves the reconnecting speed for any device regardless if the
device was paired or not.