SaltedPassword (the PBKDF2 of the password) for the last few (password, salt, iterations)
triples of the process, so a pool that reconnects, or opens several connections at once, runs
the iterations once. The key is a hash of the triple: the cache holds no password text. The
salted value is as good as the password for logging in with SCRAM, and lives as long as the
process, as the password does in the connection's Config.
SALTED_CACHE_SIZE :: 16