srm: Clean up srmRm implementation

Improves return codes and error messages.

The batching logic is changed to not be as bursty as the before.
Rather than submit a patch and wait for all to finish before
submitting the next batch, the code now submits new requests as
previous requests finish. Thus remove requests will be more
interleaved with other PnfsManager requests, and we can maintain
a high throughput with a lower limit than before. For this reason
I reduced the default value of srm.limits.remove-batch-size to 50.

Target: trunk
Require-notes: yes
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Acked-by: Dmitry Litvintsev <litvinse@fnal.gov>
Patch: http://rb.dcache.org/r/6121/