[buteo-sync-plugin-caldav] Use a time range for quick syncs

Some servers (e.g Memotoo) don't emit a proper reply if we try to
fetch the etags with a defined time range. This commit is a partial
revert of 782ef9db20125f36fec4bbf78ac04a0d335debc7.
Now the client will only store events 6 months older than current date
and 12 months in the future.