this allows an internal cache to be maintained that permits most calls to
the heavyweight getLatestSnapshotID(String store) AVM API to be skipped.
This addresses a performance problem noticed when the number of stores
is large.
git-svn-id: https://svn.alfresco.com/repos/alfresco-enterprise/alfresco/HEAD/root@6182 c4b6b30b-aa2e-2d43-bbcb-ca4b014f7261