limit constraint should be applied on storage.base.Connection

we have applied limit constraint for almost all of the major database, but get_meters and get_resources in storage.base.Connection (yes, the interface itself) have missed it. We should add limit constraint for them as well.

And there is another problem associated with the limit parameter, see:

$ ceilometer -d meter-list -q x=y

ceilometerclient.openstack.common.apiclient.exceptions.BadRequest: Unknown argument: "x": unrecognized field in query: [<Query u'x' eq u'y' >], valid keys: ['limit', 'project', 'resource', 'source', 'user'] (HTTP 400) (Request-ID: req-9881e52b-d0ee-46cd-b259-31bb9682f2ad)

the limit should not be one of the valid keys for query filter, we need to treat limit as an internal parameter, because we already use it as a parameter in API and already check it in that level.