dbwrap ctdb: add a retry loop to the persistent store operation.

dbwrap ctdb: add a retry loop to the persistent store operation.

This is because ctdbd can fail in performing the persistent_store
due to race conditions, and this does not mean it can't succeed
the next time.

To not loop infinitely, this makes use of a new parametric option:
"dbwrap ctdb:max store retries" (integer) which defaults to 5
and sets the upper limit for the number or repeats of the
fetch/store cycle.

Michael