reader: track index chunks and limit access

reader: track index chunks and limit access

a reader connection should not be allowed to read arbitrary chunks in
the datastore, but only those that were previously registered by opening
the corresponding index files.

this mechanism is needed to allow unprivileged users (that don't have
full READ permissions on the whole datastore) access to their own
backups via a reader environment.

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>