srm: fix retry of copy-pull

When srmCopy copies a file from a remote site it issues a prepareToPut and putDone
when the transfer is successful.  If the transfer is unsuccessful then an abortPut
is called, which destroys the upload directory.

The current behaviour attempts to reuse the transfer TURL when retrying.  Since
abortPut has been called, this directory no longer exists and the transfer-manager
will fail the request as it discovers this.

This patch fixes the problem by issuing prepareToPut when retrying.

Target: master
Patch: http://rb.dcache.org/r/6956/
Acked-by: Gerd Behrmann
Request: 2.9
Merge pull request #518 from paulmillar/fix/2.9/srm-copy-retry

Fix/2.9/srm copy retry