Add scopedVars and a special __time_delta entry to data available for Arithmetic.

[hakanf]

The use case is to be able to scale CloudWatch Sum or SampleCount data to the
same unit, regardless of the sample period used when performing the CloudWatch
query.

[Gauravshah]

@ShilpaSivanesan can you please review this ?

[hakanf]

After some more testing I added a few more changes:

1. Move scopedVars to the beginning of expression evaluation function arguments, since it can happen that not all data is defined in a given sample, causing scopedVars to shift back to a different argument
2. Add functionality to extract required letters from the expression, and check that those letters hold defined data before calling the expression evaluation function
3. Add a check that the sample time is within the expected window before running the expression evaluation function
4. Avoid pushing datapoint to output data if the expression evaluation function throws an error

[Gauravshah]

@hakanf please let us know when you are ready

[hakanf]

@Gauravshah , sorry, I did not understand you were waiting for me.
I have now been using this code (including the pull request) on a daily basis since doing the pull request.
The last few commits correspond to minor issues that I encountered as I was using it.
With those, I do feel ready anytime.

[Gauravshah]

thanks @hakanf