Add local cache

[lvps]

Currently, usernames are always looked up in LDAP directory, except for:

• users pass the --no-ldap parameter, or
• the standard and normalized username is passed during logout (since it is found in the log files)

This is sub-optimal and does too many queries. Before performing the search, we should look up the username (or matricola, or alias) in a cache. In case of an hit, use that result.

If the cache misses but the result is found with a LDAP search, add it to the cache.

If the LDAP search fails, do not add anything to the cache.

The cache should ideally be a dictionary, with username/matricola/alias as the key, and normalized username as the value (the uid attribute from LDAP). They may be equal: the cache still provides the information that it is a valid username. However each entry should also have a timestamp.

Entries should be invalidated and removed after a configurable amount of time, with a default of 24 hours. When accessing the cache, check all entries and remove expired ones.

The cache should be stored in ~/.cache/something as a JSON or pickled file or anything else that can be serialized and deserialized easily. The file should be created if it does not exist.