Make DirectoryString syntax the default or the server (as it has always been)

From the start the default syntax in the SDK has been OctetString, while the server used DirectoryString IIRC.
So right now, when creating DNs, if an attribute is unknown it will use OctetString instead of DirectoryString like in 2.6 .

This change brings back the default behaviour to the server. See on 2.6:
DN.decode():
https://stash.forgerock.org/projects/OPENDJ/repos/opendj/browse/src/server/org/opends/server/types/DN.java?at=refs%2Ftags%2F2.6.0#646
calls DirectoryServer.getDefaultAttributeType():
https://stash.forgerock.org/projects/OPENDJ/repos/opendj/browse/src/server/org/opends/server/core/DirectoryServer.java?at=2.6.0#4006
which delegates to DirectoryServer.getDefaultAttributeSyntax():
https://stash.forgerock.org/projects/OPENDJ/repos/opendj/browse/src/server/org/opends/server/core/DirectoryServer.java?at=2.6.0#4135
which is initialized as DirectoryString:
https://stash.forgerock.org/projects/OPENDJ/repos/opendj/browse/src/server/org/opends/server/core/DirectoryServer.java?at=2.6.0#1688