Add peek method for SortingLongCollections

[PramodRaoB]

Description of the issue:

Currently, the SortingLongCollection does not provide a method to peek the value of the top-most element. This makes it impossible to perform some processing on-top of such collections such as a k-way merge which require a non-destructive peek. Adding that feature should be trivial and not break existing code.

I could submit a quick PR if this feature is approved.

[lbergelson]

@PramodRaoB That sounds very reasonable. I will review it if you submit a PR.