OPENDJ-2638 (PR-201) Collation matching rule should be configurable via config.ldif

Collation matching rules can now be unconfigured via config.ldif and removed via dsconfig.
However new collation matching rules cannot be added via config.ldif or added via dsconfig.

CollationMatchingRuleFactory.java:
In initializeMatchingRule(), no longer throw DirectoryException
when trying to override an already existing collation matching rule
(already existing collation matching rule are hardcoded by the SDK schema).
In applyConfigurationChange(), no longer throw NullPointerException (serverContext is never set):
collation matching rules can now be removed