WALKey Extended Attributes don't serialize to ReplicationSink

HBASE-22622 introduced extended attributes on the WALKey object and protobuf, and HBASE-22623 created a coprocessor hook, preWALAppend, so that coprocessors can create and insert their own extended attributes.

These attributes are readable on the source-side, such as in a custom ReplicationEndpoint. However, in branch-1 ReplicationProtbufUtil doesn't correctly populate the extended attributes on the WALKey protobuf. (In 2.1+, HBASE-20625 incidentally fixes this as part of a larger refactoring of WALCellCodec logic.)

This means that a custom ReplicationSink can't read extended attributes on a WALKey.