Support sorting by time desc

[jwilder]

This PR adds support for sorting results by time in descending order. Two syntaxes are allowed:

select value from cpu order by time desc
select value from cpu order by desc

This allows queries to find the most recent value instead of having to query the whole data set to see the last value. Currently only sorting by time is allowed on raw and aggregate queries. If there are multiple sort fields, a parse error will be returned. Similarly, if a sort field other than time is specified, a parse error is returned.

Fixes #2022

[otoolep]

Cool.

Are all the true and false required in the tests? Are there not enums for those?

[jwilder]

Looks like I forgot to switch those over to the enum. Will fix.

[otoolep]

Should we expect a query performance hit for normal time-ascending queries? You should probably check it, if you haven't already.

[corylanou]

maybe aggregate order by time desc?

[corylanou]

Very nice. Great test coverage. +1

[jwilder]

@otoolep I'll check. Ascending should perform as before for both raw and aggregate queries. Desc raw w/ bz1 has a little bit of overhead to create an index into the compressed buffer but should not add much. Desc aggregate queries should perform identical to ascending.

[jwilder]

@otoolep Performance for raw and aggregate queries for asc and desc is the same from master to my branch using 10M points. Desc doesn't seem to have much overhead either which is good.