Add a GetDefault method.

[bn-jbowers]

This adds a GetDefault method, which will either return the value in the cache, or if it is not present, set it with the passed-in value, returning whichever value ends up in the cache.

This must be done inside the cache because calling .Get, then .Set if the value is not present, is a TOCTOU (Time Of Check to Time Of Use) problem; something else may have set that value in the meantime. Only the cache can perform this operation atomically and safely.

[Nikola-Milovic]

This seems like an useful functionality, (not a maintainer) but GetDefault usually implies a helper function that just saves you from typing if res == nil.... Would naming this GetSetDefault (or something) be more clear on what it actually does? Since it also sets the value if mising

[bn-jbowers]

I kind of copied the terminology from Python here: https://www.geeksforgeeks.org/python-dictionary-setdefault-method/

I'm ambivalent about the naming. But I'm going to end up forking this internally now, since I need this. (For my use case it is concurrently-incorrect without this.)

[Code-Hex]

@bn-jbowers Thanks! I cherry-picked your commit at #49