fix compile error on wrong import package and add missing (c) header
Use guava instead of custom iterable and iterator decorators in mongo storage.

Replace AdaptingIterable/Iterator and ConcatenatedIterable/Iterator
by their corresponding guava facilities as used in the rest of the code
Move MongoGraph to package org.geogit.storage.mongo

There's no need for MongoGraph to be on its own package 'blongo'. Better
to favor package cohesion and lower coupling.