fix(wsgi): Cleanup limit config options

This patch contains several misc. changes to queue, message, and
claim limits to reduce confusion and bring the implementation in
line with the v1 spec.

1. Removed a couple of WSGI driver config options that are
no longer needed now that we have redefined (and simplified) how
we constrain message and metadata size.

    metadata_max_length = 65536
    content_max_length = 262144

2. Renamed options to be more readable and consistent
3. Moved options to [transport] section
4. Made max messages that can be claimed its own setting, to reduce confusion
5. Removed enforcing an upper limit on the number of messages that can be
posted; this was never in the spec, and appears to be gold-plating. Now, the
only upper limit is max_message_size.
6. Removed the check on the size of a create claim request since (1) it is
not part of the API spec, and (2) sanity-checks like that are best done by
the web server, before a request even touches the app.
7. Migrated limits for storage driver interface params to static values,
since those defaults define the static contract between transport and
storage drivers.
8. Wrapped validation error messages in gettextutils._, and converted them
to use .format instead of %.

Change-Id: I1372e5002f030f5c8c47774ab00ca8ee7e12232d
Closes-Bug: #1270260