Don't give checkers pointers to CouchbaseCluster

Currently the signature of a checker function is

type CheckerFn func(cluster *CouchbaseCluster) ([]*WrappedCheckerResult, error)

This, in theory, means that any checker could mutate the CouchbaseCluster, which means that (in theory) the checker functions aren't thread-safe and can't be called concurrently.

While there shouldn't be a reasonable scenario where they would mutate it, we should still look into the viability of changing this to cluster CouchbaseCluster, ensuring they can't mutate the underlying source data.