Store hostname hashes in memory rathen than the non-hashed host entries. Also assures that the host entries in known_hosts get saved in hashed format as it is currently standard in OpenSSH.
Assure that host entries in known_hosts files do not duplicate endlessly if keys from known_hosts are loaded via HostKeys.load() more than once (e.g. for refreshing the list of known hosts during runtime).
Load host entries from the known_hosts file(s) before writing the file from RAM to disk. Avoids loss of host entries in case other SSH clients have written to the known_hosts file(s) meanwhile.
do not write ,,garbage'' to known_hosts file(s)
Changelog re #87
Back out broken but non-required hostname hash change