BibSched: multi-node support

• Allows multiple bibsched instances (on multiple nodes) to access the task queue and to run possible per-node jobs. A new configuration variable CFG_BIBSCHED_NODE_TASKS governs this behaviour.

• Amends BibSched task monitor to show nodes where tasks are executed.

• Uses atexit in order to unlock tables; this is useful in cases bibsched or other Invenio processes die.

• Enlarges schTASK.proc column in order to allow for processes with longer names.