hsm: Increase concurrency in nearline storage interface

When receving many HSM requests at once (in particular stage), the strict
synchronization in the nearline storage interface causes these requests to
serialize. In particular for stage requests this may be a problem, as the
replica entries are created in this phase.

This patch increases the concurrency in the nearline storage interface.

Target: trunk
Require-notes: yes
Require-book: no
Acked-by: Albert Rossi <arossi@fnal.gov>
Patch: https://rb.dcache.org/r/7183/