Get rid of "cache" concept

[tlater-famedly]

The "cache" is a holdover from ldap-poller, and used to keep track of what changed since the last run of the tool, so that we know when to send update and deletion requests instead of import requests.

While this works for now, and is fine as long as ldap-sync is the only tool performing syncs and the "cache" is persisted, the databases will go out of sync the moment this is not true.

The name "cache" is therefore a big misnomer, since the name implies it can be safely removed at a performance penalty, when in reality doing so breaks the tool. We also currently don't have a good way to get things back into sync when issues (e.g. failed requests) occur because ldap-poller doesn't have an interface to bust the "cache".

We should move away from this.

[tlater-famedly]

Waiting for famedly/zitadel-rust-client#15