Fix for issue 2505 (Provide monitoring information about the number of entries on a given base DN)

The following changes are aimed to include a new attribute in the backend monitor entry to provide the number or entries per base DN. This information is used by the status, status-panel, dsreplication and graphical setup to be able to display the number of entries under a given base DN even when there are several base DNs defined in the same database.

The way the new attribute appears in the monitoring entry is:

ldapsearch -w s -p 1389 -b cn=monitor ds-backend-id=userRoot ds-base-dn-entry-countdn: cn=userRoot Backend,cn=monitor
ds-base-dn-entry-count: 0 dc=ta
ds-base-dn-entry-count: 1 dc=ti
ds-base-dn-entry-count: 1 dc=to

I prefer this approach because in my opinion having the number of entries is more important that the suffix.  Anyway I think the discussion around the structure of the value is not crucial as long as the value is easy to parse (which is the case).

The new attribute is defined in the schema and the new OID assigned to it is 1.3.6.1.4.1.26027.1.1.434.

In order to have a minimum impact on performances, the method backend.numSubordinates is only called when there are several base DNs defined in the database.