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.
Given that the transient data details are probably of a fixed size (eg, the presence ones) I guess this means that we shouldn't see exhaustion thrash often, since the value blocks (serialized presence detail blobs) should be the same size?  Or am I misunderstanding?