Implement rule to convert filter to range when possible

[wolffcm]

If a user enters a query like

from(bucket: "telegraf") |> filter(fn: (r) => r._time >= now() - 1h)

It will fail planning because there is no range to push down. However, the filter is semantically equivalent to a range and should be rewritten as such:

from(bucket: "telegraf") |> range(start: -1h)

The rule should be made to handle reasonable expressions using relational operators and the _time column.

This would be a logical rule defined in filter.go.

[nathanielc]

Note, that currently the binary operators like == and >= are not defined for time values. This will need to address that.

[wolffcm]

range will create _start and _stop columns, and filter does not. Should the rule also generate a drop node to drop these columns?