All caracteristics shall contain a value in the next handle so having
a declaration on 0xffff is invalid since there won't be any space for
the value descriptor.

The spec state that a secondary needs to referenced by at least one
primary, Version 4.2 [Vol 1, Part A] page 101:

  'A secondary service is a service that provides auxiliary
  functionality of a device and is referenced from at least one
  primary service on the device.'

This makes notify_client_ready takes care of setting ready flag so the
caller code don't have to set it before calling notify_client_ready.

This rework the code using service changed registration so it becomes
reusable by different part of code.

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.