core: Refactor memory allocation for GATT service discovery

Prior to implementing partial primary service discovery, it is necessary
to move allocation of struct included_search, so the list of found
services (search->services) can be created incrementally.

Note that this list is also used later to collect (secondary) included
services and only then the full list of found services is used for
registering new services and detecting added/removed profiles.
core: Do not pass around memory allocated internally by GATT API

Instead, do a "deep copy" of the list of GATT services. While this
generates extra heap allocations, it also allows proper cleanup in case
of ATT or link errors. In other words, memory allocated by GATT API will
be freed by GATT API after callback returns.
WIP