Update to io.grcp 1.21.0 and protobuf 3.7.1

Include 4.1.35.Final netty bundles in maven external dependencies to
keep netty in the same version that is included by GRPC 1.21.0 in the
third-party repository.

Also removed org.apache.xmlbeans and com.github.virtuald.curvesapi from
external maven dependencies since they are only needed by Apache POI
3.15.0 which is included in the manual external dependencies in the
third-party repository.

gitlab #308

Change-Id: I8c7eb2a14f4df585e735540945ade8a18859cb6a