[pdatagen]Introduce method to reslice for pcommon.UInt64Slice

[lahsivjar]

Is your feature request related to a problem? Please describe.

The current interface exposed by pcommon.UInt64Slice for the underlying slice leads to a lot of allocations for use-cases which deals with datastructure like histograms or exponential histograms. For example, merging two exponential histograms almost always requires a new slice to be allocated even if the conditions are met to allow reusing one of the slices.

Describe the solution you'd like

Probably not the ideal solution but while working on optimizing merges for exponential histograms I wanted to have access to a method that would allow me to reslice the underlying slice - a method like Reslice(lower, upper int) which would basically do slice[lower:upper] on the underlying slice.

I am not aware with all the design intentions for the pdata, so does it make sense to have a Reslice method?