Replace SHOW/Describe and internal schema with rewrite, and database

[araddon]

The SHOW/Describe code is not great. Replace it with query-rewrite, so that show statements become normal Select statements that operate against normal data sources.

• frontends (such as mysql) need to be able to over-ride default behavior of "Writers" so that dialect specific is builtin

TODO

• implement more of the schema in normal datastore type backends
• the plan.Context will need to use a different schema from connection, ensure the plan.Context.Schema is used not any connection specific ones throughout.
• rewrite show statements to select statements to utilize the build-select which has better where/filter support
• WHERE/LIKE filters : variables like '%xyz' add where-clause support for show/describe
• variables (implement session)
• ensure all downstream show, describe's work in dataux
  • fix horrible datasource/registry
• resolve the new required schema dependency issue that breaks literal queries and unit test in plan