Added clirr-maven-plugin to catch non backward compatible changes in opendj-core compared to opendj-ldap-sdk so they are documented.

pom.xml:
Now depends on forgerock-parent 1.2.1-SNAPSHOT to inherit clirr profile.

opendj-core/pom.xml:
Added clirr-maven-plugin to the build and report plugins due to the use of <comparisonArtifacts> instead of <comparisonVersion>.

opendj-core/clirr-ignored-api-changes.xml:
Added to trigger clirr checks + ignore and document changes compared to opendj-ldap-sdk.