OPENDJ-1231 Make the Medium Consistency Point support replica heartbeats

Christophe reported to me that functional tests with the changeNumber were failing.
After investigation on my side, it turns out that the mediumConsistencyCSN was blocked on a "cn=admin data" or "cn=schema" CSN.
Fix consists in excluding these domains in ChangeNumberIndexer's publishHeartbeat() and publishUpdateMsg() methods.


ChangeNumberIndexer.java:
Added isExcludedFromECL() + used it from publishHeartbeat() and publishUpdateMsg() to exclude undesired domains.