Implement permutation of EnumerateCollectionNodes.

Also: restructure leveling of rules.
And:  remove some debugging output (and add some other).
Adjust the output level of useSortIndex rule.
Merge branch 'aql2' of ssh://github.com/triAGENS/ArangoDB into aql2

Conflicts:
	arangod/Aql/Optimizer.cpp
Rearrange levels for optimizer rules. Use rules multiple times.