[qtcontacts-sqlite] Match to existing aggregate in SQL

Reading all aggregates into memory for match testing is not workable
for any reasonable number of contacts.

The match algorithm is now less sophisticated, but if necessary it
can be extended to correspond to the old version.
[qtcontacts-sqlite] Index on fields used in searches
[qtcontacts-sqlite] Use dedicated columns for lowered versions of text values

Conversion to lower inside queries is incredibly expensive, and may
also defeat indexing?