Add support for Graphite web compressPeriodicGaps function

[carrieedwards]

Is your feature request related to a problem? Please describe.
CarbonAPI is currently missing support of the compressPeriodicGaps function. This function is defined as:

compressPeriodicGaps(seriesList)
Tries to intelligently remove periodic None’s from series, recalculating start, stop and step values. You can use summarize(seriesList, ‘<desired step>’, ‘last’) function for that also, but this function trying to guess desired step automatically. Can be used in case of fix metric with improper resolution. Especially useful for derivative functions, which are not working with series with regular gaps.

Example:

&target=compressPeriodicGaps(Server.instance01.threads.busy)

Graphite web's implementation can be found here.

Describe the solution you'd like
This function should be supported and thoroughly tested to ensure that it is functioning according to expectations.

Additional context
Add any other context or screenshots about the feature request here.