OAK-1332: Large number of changes to the same node can fill observation queue

Postpone the construction of event paths and identifiers to when they are
actually requested. Currently this requires holding references to the
ImmutableTree instances and thus quite a bit of extra memory, but further
refactoring should make it possible to avoid the tree references.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1561247 13f79535-47bb-0310-9956-ffa450edef68