Update the backend API to include new isIndexed methods that can be used to
determine whether a given attribute is indexed in a specified manner, or that
can be used to determine whether a specified filter is indexed.  At present,
all backends except the JE backend and the replication backend are considered
always indexed for all operations.  The JE backend is only considered indexed
based on its index configuration.  The replication backend is never considered
indexed.

Update the following components to make use of this new isIndexed capability:
- The DSEE-compatible access control handler will now log a warning message at
  startup if it detects that there is no presence index for the aci attribute,
  which can make startup take a long time on a big database.
- The group manager will now log a warning message at startup if any of the
  group implementation filters are unindexed, which can make startup take a
  long time on a big database.
- The referential integrity plugin now requires that all of the attributes for
  which referential integrity is to be maintained must be configured with
  equality indexes.
- The unique attribute plugin now requires that all of the attributes for which
  uniqueness is to be enforced must be configured with equality indexes.

This commit also updates the LDIF backend so that it is possible to
indicate via configuration whether its base DNs should be registered as public
or private base DNs.  The LDIF backend used as the admin root has been
updated so that it is considered a private backend.  The replication backend
has also been updated so that it is considered a private backend.