[qtcontacts-sqlite] Use immediate locking mode for transactions

Since we already use exclusive locking via IPC, there will be no penalty
using immediate lock acquisition mode for transactions.  This appears to
fix the issue where readers spuriously report 'database disk image
is malformed' during writes by other processes.
Merge pull request #56 from matthewvogt/immediate-locks

[qtcontacts-sqlite] Use immediate locking mode for transactions