srm: Refactor persistence architecture

Addresses several issues related to persistence of SRM requests:

- Non-strict layering of JobStorage: Jobs could be loaded either from
  a JobStorage or through static methods in the Job class. The latter
  would canonicalize job instances and implement sharing throughout
  a terracotta cluster and it was thus essential that the Job methods
  were used. Yet, in several places the JobStorage would be accessed
  directly.

  This problem is resolved by implementing the shared memory cache and
  canonicalization code through JobStorage decorators, thus enforcing
  strict layering.

- Use of non-bulk loading of jobs during startup. Since the static
  methods of the Job class only supported loading one job at a time,
  restarting an SRM would be very slow if the database contained
  many non-final jobs.

  This is addressed by relying on bulk operations instead. Since
  shared memory caching and canonicalization are now implemented as
  decorators to JobStorage, bulk operations are now "safe" to use as
  they are intercepted and processed by the decorators too.

- Several pieces of code would cast JobStorage to DatabaseJobStorage
  to perform special queries, e.g. SrmReleaseFiles. This lead to
  several problems: If database persistence is disabled or configured
  to only store requests in a final-state, the type cast would not
  be possible and thus said functionality would no longer work. The
  cast also bypassed the shared memory cache and canonicalization,
  and finally the data read from the database may be stale as updates
  are written asynchronously.

  This is addressed by extending the JobStorage interface with
  an operation to fetch non-final jobs, thus allowing these jobs
  to be obtained without casting the JobStorage instance and without
  bypassing the canonicalization and caching layers.

- During restart, only unscheduled pending jobs would be loaded back
  from the database. Thus code that relied on being able to fetch
  active jobs from the caching layer would not work after a restart.
  In particular the SRM_FILE_BUSY logic would no longer detect busy
  files after a restart.

  This is addressed by populating the cache with all non-final jobs
  during restart. As before, only unscheduled pending jobs are scheduled
  for further execution - this can certainly be improved, however
  for the purpose of implementing SRM_FILE_BUSY and for properly
  releasing TURLs at the end of the transfer, the current logic
  suffices.

- Canonicalization relied on a WeakHashMap, which meant that the particular
  instance of the request ID (a Long) was significant for the caching. If
  that particular Long would no longer have a reference, the entry
  would be released.

  This is addressed by using a map with weak values created by Guava. This
  also means that in a subsequent patch we can replace all those Long IDs
  with long.

Target: trunk
Request: 2.7
Require-notes: yes
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/5929/
(cherry picked from commit 9650be1f8ee50876a58fc116c1ad258b240ee274)
Merge pull request #177 from gbehrmann/fix/2.7/rb5929

srm: Refactor persistence architecture