* Move batch.Importer interface to pilosa.Importer
In addition to moving the interface, it updates all the methods to use
dax.TableID (for example) intead of a string pilosa index name.
* Change unused onPremImporter methods to no-op.
onPremImporter is a wrapper around API which implements the Importer
interface. This is currently only used by sql3 running locally in standard
(i.e not "serverless") mode. Because sql3 always sets
`useShardTransactionalEndpoint = true`, There are several methods which this
implemtation of the Importer interface does not use, and therefore they
intentionally no-op.
(cherry picked from commit 12d608c80d)
* Thread Table.Description through SchemaAPI
* Thread Table.CreatedAt through SchemaAPI
* Thread Field.CreatedAt through SchemaAPI
(cherry picked from commit 734477aaee)
* WIP: Convert SchemaAPI to be DAX-centric
* Tables(), CreateField()
* CreateTable(), DeleteTable(), DeleteField()
* More cleanup
* Remove the old SchemaAPI
(cherry picked from commit a15783cb49)
