Add support for Graphite web identity function

[carrieedwards]

Is your feature request related to a problem? Please describe.
The identity() function is implemented in Graphite web, but is missing from CarbonAPI. In order to enable users to be able to use the identity() function, it needs to be implemented in CarbonAPI.

Describe the solution you'd like
The identity() function should be implemented in CarbonAPI, with the same functionality as the Graphite web function. The description of the function is as follows:

identity(name)
Identity function: Returns datapoints where the value equals the timestamp of the datapoint. Useful when you have another series where the value is a timestamp, and you want to compare it to the time of the datapoint, to render an age

Example:

&target=identity("The.time.series")
This would create a series named “The.time.series” that contains points where x(t) == t.

Additional context
Tests should be added to verify that the functionality matches that of the Graphite web identity() function.