Implemented the tag values iterator for SHOW TAG VALUES

[jsternberg]

Fixes #5593.

[jsternberg]

I had to add another filtering function which was a bit weird and I would prefer another solution with more code reuse, but I couldn't think of a way that didn't break another part of functionality.

[jsternberg]

Not quite done yet. It still needs to handle more functionality that I didn't realize needed to be covered. This kind of call has to be supported: SHOW TAG VALUES WITH KEY = host WHERE region =~ /ca.*/

[jsternberg]

I think it's working now.

@jwilder @e-dard @benbjohnson please review. I'm not sure how happy I am with the end result, but I do believe it works. This change also affects backwards compatibility so I would like any feedback regarding that. The change to the output is located in the commit message.

[benbjohnson]

Can you make a RewriteExpr() that accepts and returns an Expr so that you don't have to do the nil cast everywhere?

[jsternberg]

That sounds like a good idea. I can do that.

[benbjohnson]

Overall it lgtm. Can you post a before/after example of the output and verify with @pauldix about the change. I'm ok with it.

[jsternberg]

Change in output from:

> SHOW TAG VALUES WITH KEY IN (host, region)
name: hostTagValues
-------------------
host
server01
server02

name: regionTagValues
---------------------
region
useast

to

> SHOW TAG VALUES WITH KEY IN (host, region)
name: cpu
---------
key     value
host    server01
region  useast

name: mem
---------
key     value
host    server02
region  useast

[pauldix]

new output format looks good to me. Does it support SHOW TAG VALUES FROM cpu WITH KEY IN (host, region)?

@mark-rushakoff: this change will almost certainly break Chronograf so we'll have to update for it.

[jsternberg]

@pauldix yep, it supports that.

> insert cpu,host=server01 value=2
> insert mem,host=server02 value=3
> show tag values from cpu with key = host
name: cpu
---------
key     value
host    server01

> show tag values with key = host
name: cpu
---------
key     value
host    server01


name: mem
---------
key     value
host    server02