Add time range extraction and interval grouping support to execution engine.

## Overview

This pull request adds support for extracting the time range from an expression and then applying that to the interval synchronization within the processors. tl;dr the start and (optionally) end time range of the query is extracted and used for the iterators passed to each mapper so they can emit values on the same intervals (even if there aren't actually data points in those intervals).

## Example

This PR includes support for interval group by statements with a time range:

```
SELECT sum(value)
FROM cpu
WHERE time >= now() - 3h AND time < now()
GROUP BY time(30m)
```

## Notes

There will be more changes once full group by support is added (e.g. n-number of tag dimensions).

I moved some notes I made with Paul to the `NOTES` file. I'll remove it once the execution engine is done. Don't bother reading through it. It won't make any sense. :)
This section tightens up all ranges so if you have:

```
WHERE time > now() - 2h AND time > now() - 1h AND time < now()
```

Then the time range will tighten the time range to:

```
WHERE time > now() - 1h AND time < now()
```

We also want to normalize the min/max so we can do GTE/LTE checks in the iterator instead of also checking for LT/GT checks. Since we have a time precision of `1µs` then we can simply increment/decrement a LT/GT check by `1µs` and turn it into a GTE/LTE check.