TS-1728: Assign storage entries to volumes

Introduces the volume assignment in storage.config, on an optional
basis. No change is needed in the config if you don't want to use
this feature.

* Add an optional reference to the volume number in storage.config,
  and the Span object, in Store.h and Store.cc
* Add the volume reference to DiskVol, CacheProcessor::start_internal
  (in Cache.cc)
* Change cplist_reconfigure (in Cache.cc) to constrain mapping
* Fix bytes_used and percent_used per volume stats
Merge branch 'issues/TS-1728' into 'master'
TS-1914: configured socket buffer sizes not applied