Improve the 409 Conflict detection, address DB concurrency.

The conflict now happens only if under the same withdraw
operation ID the wallet tries to select two different exchanges
or reserve public keys.

As of DB concurrency, there is now one thread (named "DB")
that should run all the database operations, in order to avoid
conflicts on the disk.  At this moment, and mostly to see where
the current implementation fails with regard to concurrent DB
access, not all the database operations were migrated into such thread.

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