backupccl: distribute RESTORE work using DistSQL

This commit replaces restore's old method of coordinating work on a
single node to instead use DistSQL. It creates a 2 stage DistSQL flow.
The first stage assigns chunks to SplitAndScatter processors which
forward spans that they scattered to the second stage made up of the
RestoreData processors which ingest the data. The SplitAndScatter
processors attempt to send the work to the RestoreData colocated with
the leaseholder of the span after the scattering.

Release note: None.

pkg/ccl/backupccl/backup.proto

@@ -175,4 +175,5 @@ message ScheduledBackupExecutionArgs {
 message RestoreProgress {
   RowCount summary = 1 [(gogoproto.nullable) = false];
   int64 progressIdx = 2;
+  roachpb.Span dataSpan = 3 [(gogoproto.nullable) = false];
 }

pkg/ccl/backupccl/restore_data_processor.go

@@ -159,6 +159,7 @@ func runRestoreData(
 		progDetails := RestoreProgress{}
 		progDetails.Summary = countRows(importRes.(*roachpb.ImportResponse).Imported, spec.PKIDs)
 		progDetails.ProgressIdx = entry.ProgressIdx
+		progDetails.DataSpan = entry.Span
 		details, err := gogotypes.MarshalAny(&progDetails)
 		if err != nil {
 			return err