s3: add dbwrap_torture - a tool to stress test tdb transactions through dbwrap

s3: add dbwrap_torture - a tool to stress test tdb transactions through dbwrap

This can be used to also test tdb transactions on clustered installations
throught ctdb. The test is modeled after the ctdb_transaction.c test program
from the ctdb source code. It runs transactions in a tight loop on a test
database called "transactions.tdb" (by default), increasing a counter in each
iteration. In a clustered environment, a counter is maintained for each node.

Michael