s3:dbwrap_ctdb: fix a race in starting concurrent transactions on a single node

s3:dbwrap_ctdb: fix a race in starting concurrent transactions on a single node

There are two races in concurrent transactions on a single node.
One in starting a transaction and one with replay during commit.

This commit closes the first race by storing the client pid in the
transaction-lock record and comparing the stored pid against its own
pid after releasing the lock and refetching the record inside the
transaction.

Michael