Selective sync: Skip excluded folders when reading db

Selective sync: Skip excluded folders when reading db

When a new folder becomes selective-sync excluded, we already mark it
and all its parent folders with _invalid_ etags to force rediscovery.

That's not enough however. Later calls to csync_statedb_get_below_path
could still pull data about the excluded files into the remote tree.

That lead to incorrect behavior, such as uploads happening for folders
that had been explicitly excluded from sync.

To fix the problem, statedb_get_below_path is adjusted to not read the
data about excluded folders from the database.

Currently we can't wipe this data from the database outright because we
need it to determine whether the files in the excluded folder can be
wiped away or not.

See owncloud/enterprise#1965