Enhancement: Cache basic authentication credentials

To speed up the verification of basic auth credentials, rest-server now caches
passwords for a minute in memory. That way the expensive verification of basic
auth credentials can be skipped for most requests issued by a single restic
run. The password is kept in memory in a hashed form and not as plaintext.

https://github.com/restic/rest-server/issues/133
https://github.com/restic/rest-server/pull/138