cache: refactor, and fix of race condition between expiration goroutine and external calls.

[colindickson]

Solves #117

[colindickson]

Notes:

Created private "lock-free" versions of the Add/Delete/Get/Len methods to be called internally. This way, the goroutine that is running which deletes expired items can acquire a write-lock on the map and do all of its work using these methods without worrying about a concurrent operation on the map.

[samcm]

Thanks @colindickson!!

[samcm]

Any chance you could look at the linting errors @colindickson? I'll merge + release once we're good to go :)

[colindickson]

@samcm should be good now. :)

[samcm]

Included in https://github.com/ethpandaops/checkpointz/releases/tag/v0.18.0! Thanks again @colindickson 🎉