Simplify 'invalid cashes' verification

Instead of checking cachify prefix against the md5 hash of the file content we just check if the said prefix is among the ones that we previously generated.

It provides a safeguard against someone maliciously attempting to clog the downstream cache by sending us a great number of invalid URLs and has an added benefit of beeing much simpler than 'recalculate the hash' method.

Fixes #37