Update group checksums when combining multiple deltas. (#5394)

The function processing the oracle delta stream combines multiple deltas
into one to reduce the number of proposal. However, the group checksums
were not being updated, causing some group checksums update to be lost.

gRPC guarantees that the ordering of the deltas coming from the stream
so overwriting the group checksums is safe.

Tested by following the steps in #5368.

Fixes #5368

worker/groups.go

@@ -947,6 +947,9 @@ func (g *groupi) processOracleDeltaStream() {
 					batch++
 					delta.Txns = append(delta.Txns, more.Txns...)
 					delta.MaxAssigned = x.Max(delta.MaxAssigned, more.MaxAssigned)
+					for gid, checksum := range more.GroupChecksums {
+						delta.GroupChecksums[gid] = checksum
+					}
 				default:
 					break SLURP
 				}