Fix for issues 593, 1045 and 1060.

593: attribute indexes need to support subtypes and options
1045: possible index corruption (subtree search fails to return matching entry)
1060: entry disappears when modifying with ldapbrowser

There were bugs in the way an equality index was updated for a modify operation.  Trying to determine the index changes from the sequence of modifications did not work in the general case.  Now, it ought to be correct in all cases.

I've changed many of the modify operation unit tests to run against the JE backend instead of the memory backend.  Also I changed the tests to modify the mail attribute instead of description, because description is not indexed.