core: Initialize GATT attribute database before adapter

The GATT attribute database will be shared between all adapters, thus it
must be available before the first adapter is initialized.
gatt: Add function for iterating over attributes on local database

This function will allow to import the new database into the old one,
allowing for incremental migration to the new GATT API.
core: Import GATT attributes from new database into old one

This will allow to incrementally migrate current GATT server profiles to
the new API.

After this commit, GAP/GATT services will be served from the new
database.