Make protobuf fields add post-2.0.0 release marked 'required' instead be 'optional' so migrations from 2.0.x to 2.1+ or 2.2+ succeeds.
This module has proto files used by core. These protos overlap with protos that are used by coprocessor endpoints (CPEP) in the module hbase-protocol. So core versions have a different name, the generated classes are relocated -- i.e. shaded -- to a new location; they are moved from org.apache.hadoop.hbase.* to org.apache.hadoop.hbase.shaded.