WIP: [qtcontacts-sqlite] Optionally allow non-durable updates

Unfinished - to reduce I/O load, allow presence updates to be applied to a shared memory buffer, rather than to the database itself.  These updates will not be durable, which is an acceptable trade-off in this situation.
No, the likely content for a contact will be a list of all details ot types (Presence, GlobalPresence, OnlineAccount, OriginMetadata).  So, it will depend on how many accounts a contact has, and then on how much data they have for any given account.  But QCOA is comparatively large, so I would expect something on the order of a couple of hundred bytes rather than less.