Align binary math expression streams by time

[jsternberg]

Also fills in missing values using the fill expression for any binary
aggregation.

Fixes #7427 and #7469.

[jsternberg]

@mark-rushakoff this can act as an alternative to #7464 that also implements fill() for missing values. I'll also cherry pick the test case that you have in that PR so we can have it here too.

[mark-rushakoff]

Would it be better to have this be an array rather than a slice, so that the compiler can enforce length of 2?

[jsternberg]

I tried that at first, but then I can't set it to nil when fill(none) is specified. Oh, that reminds me. I forgot to implement fill(previous) which is the only reason this is here to begin with.

[jsternberg]

Should this be the correct output of fill(previous)?

> select a, b, a + b from cpu fill(previous)
name: cpu
time                    a       b       a_b
----                    -       -       ---
1476729651246087386     1
1476729656794710992             2       3
1476729661978670263     3       4       7

[desa]

@jsternberg wouldn't we get something like

> select a, b, a + b from cpu fill(previous)
name: cpu
time                    a       b       a_b
----                    -       -       ---
1476729651246087386     1
1476729656794710992     1       2       3
1476729661978670263     3       4       7

or am I just misunderstanding?

[jsternberg]

That's a good point, but I'm not sure I can do that as part of this PR since it's a separate part of the code. If we think that's a proper thing to do in the future, we might want to add the binary functionality now (like this PR) and then make a note in the docs that we're going to do it for the other types of queries too so it does the output that you said.