unit/test-gatt: Use gatt-db for CLIENT tests

This patch rewrites parts of unit/test-gatt that verify the contents
of a shared/gatt-client to use the client's gatt-db instead of the
service iterators.
shared/gatt-client: Remove GATT structs and iterators

This patch removes the high-level structs and iterators used by
shared/gatt-client in favor of gatt-db.