Add support for TS.REVRANGE and TS.MREVRANGE

[gkorland]

This command is exactly like TS.RANGE and TS.MRANGE, but with the notable difference of returning the entries in reverse order, and also taking the start-end range in reverse order:

[filipecosta90]

@gkorland @K-Jo as for your tsbs requirements request:
tsbs q15 groupby-orderby-limit:

The last 5 aggregate readings (across time) before a randomly chosen endpoint

could make usage of this.

[filipecosta90]

@gkorland @K-Jo for q15 and example of the real query would be:

TS.MREVRANGE {TIMESTAMP} - AGGREGATION max 60000 FILTER measurement=cpu COUNT 5

[filipecosta90]

@gkorland @K-Jo will implement this on an experimental branch