[qtcontacts-sqlite] Do not lock the write mutex during startup

Locking the write access mutex is no longer required to test if we
are the initial connection.  Locking allows reader processes to become
stuck during startup if an extended write is taking place.
[qtcontacts-sqlite] Prefer quick_check to integrity_check

Instead of performing a full integrity check when opening the database,
perform the 'quick' check, since the full check becomes extremely
slow when there is a large amount of data.