Allow multiple weighted JWDs per object store backend

[bgruening]

A typical object store backend looks like the following:

        <backend id="files6" type="disk" weight="1" store_by="uuid">
            <files_dir path="/data/6/galaxy_db/files"/>
            <extra_dir type="temp" path="/data/6/galaxy_db/tmp"/>
            <extra_dir type="job_work" path="/data/jwd/main"/>
        </backend>

To distribute the JWD load more efficiently we have multiple SSD tanks. It would be nice if we also could have some kind of balancing between JWDs belonging to one backend. Something like the following would be great:

        <backend id="files6" type="disk" weight="1" store_by="uuid">
            <files_dir path="/data/6/galaxy_db/files"/>
            <extra_dir type="temp" path="/data/6/galaxy_db/tmp"/>
           <extra_dirs>
                <extra_dir type="job_work" weight="1" path="/data/jwd/main"/>
                <extra_dir type="job_work" weight="2" path="/data/jwd/main"/>
            </extra_dirs>
        </backend>

In addition, it would be nice to have maxpctfull in place, where an admin can specify a max usage percentage for JWD.

        <backend id="files6" type="disk" weight="1" store_by="uuid">
            <files_dir limit_percent="90" path="/data/6/galaxy_db/files"/>
            <extra_dir type="temp" path="/data/6/galaxy_db/tmp"/>
            <extra_dirs maxpctfull="90">
                <extra_dir type="job_work" maxpctfull="90" weight="1" path="/data/jwd/main"/>
                <extra_dir type="job_work" maxpctfull="80" weight="2" path="/data/jwd/main"/>
            </extra_dirs>
        </backend>