rgw: Add 'radosgw-admin mksnap' command, implemented by RGWSnapshot->make()

It doesn't actually snap the rados pools, but it does build a list of
pools to snap.  The actual snapshots are next.
rgw: radosgw-admin  mksnap works, but it doesn't check for existing snapshots, nor fail well when there is a mix of RGW and Rados snapshots on the pools.
rgw: RGWSnapshot::get_snapshots() now returns a list of existing snapshots, so that RGWSnapshot::make() can detect existing snapshots.

Removed some debugging, and added warnings about mixing radosgw-admin mksnap and rados mksnap on the RGW pools.
rgw: Forgot to make static functions static.
rgw: Add 'radosgw-admin lssnap', which calls RGWSnapshot::get_snapshots()
rgw: Clean up some debugging.  Add some overall comments at the top of RGWSnapshot
rgw: Forgot to add 'lssnap' to Usage.
rgw: Rename RGWSnapsoht::make to RGWSnapshot::create, to better match the IoCtx snapshot functions.
rgw: Add 'radosgw-admin rmsnap', implemented by RGWSnapshot::remove()
rgw: Add rmsnap to radosgw-admin --help.  Moved --snap arg to a better section in --help.
rgw: Fix unit tests for radosgw-admin --help
rgw: Code review changes: remove redundant #include's