WIP: RethinkDB for blaze

- [ ] `into`
- [x] `create_index`
- [x] `drop`
- [ ] `Join`
- [x] `Map` (needs #519)
- [x] `discover`
- [x] `By`
- [x] `Summary`
- [x] additional reductions such as `std`, `var`
- [x] `ColumnWise`
- [x] difference between `t[[column]]` and `t[column]`? no, the way to select "columns" is with `pluck` which has no notion of the difference between a single column table and a single column.
- [x] use `RqlQuery` everywhere except a few places where a `Connection` is needed
- [x] check out `post_compute` uses the `Connection`

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