[Feature Request] Cache interface is not extensible

[bgavrilMS]

The current cache interface is not extensible enough. Adding new features to the cache is difficult.

At a minimum, the interface should be changed to allow modification.

Note: this proposal is somewhat orthogonal to #379, but #379 does show the difficulty of evolving the interface.

Existing interface

type ExportReplace interface {
	Replace(cache Unmarshaler, key string)
	Export(cache Marshaler, key string)
}

Proposal 1

type ExportReplace interface {
	Replace(cache Unmarshaler, hints CacheHints)
	Export(cache Marshaler, hints CacheHints)
}

Where CacheHints is an object. Adding extra properties to objects allows the interface to evolve.
Currently this object would have 1 property, key string (I suggest renaming it, as key is a bit generic, in .NET use SuggestedCacheKey).

In the future we can have: hasTokens boolean, suggesedExpiryDate datetime

Proposal 2

Same as proposal 1, but switch to a Read/Write/Delete interface + context object. Export / Replace seems more cryptic.

[bgavrilMS]

CC @pmaytak for the Marshaler / Unmarshaler abstraction used here.