[cuckoocache] Use matchKey instead of the == operator.

Summary:
We plan to allow for Element to be a key/value pair in order to store informations in the cache. This means simple equality cannot be used anymore.

A wrapper type is provided to add the matchKey feature to any type conveniently.

Test Plan:

make check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4641