GC Txn entries through GC queue

see cockroachdb#2062.

on each run of the GC queue for a given range, the transaction
and sequence prefixes are scanned and the following actions taken:

* old pending transactions are pushed (which will succeed), effectively
  aborting them
* old aborted transactions are added to the GC request.
* aborted and committed transactions will have the intents referenced
  in their record resolved synchronously and are GCed (on success)
* sequence cache entries which are "old" and belong to "old" (or
  nonexistent) transactions are deleted.

roachpb/data.go

@@ -666,8 +666,7 @@ func (t Transaction) Short() string {
 // nanoseconds since the Unix epoch.
 func NewGCMetadata(nowNanos int64) *GCMetadata {
 	return &GCMetadata{
-		LastScanNanos:     nowNanos,
-		OldestIntentNanos: proto.Int64(nowNanos),
+		LastScanNanos: nowNanos,
 	}
 }
 
@@ -713,6 +712,16 @@ func (l Lease) OwnedBy(storeID StoreID) bool {
 	return l.Replica.StoreID == storeID
 }
 
+// AsIntents takes a slice of spans and returns it as a slice of intents for
+// the given transaction.
+func AsIntents(spans []Span, txn *Transaction) []Intent {
+	ret := make([]Intent, len(spans))
+	for i := range spans {
+		ret[i].Span, ret[i].Txn = spans[i], *txn
+	}
+	return ret
+}
+
 // RSpan is a key range with an inclusive start RKey and an exclusive end RKey.
 type RSpan struct {
 	Key, EndKey RKey

roachpb/data.proto

@@ -295,12 +295,11 @@ message Lease {
 
 // GCMetadata holds information about the last complete key/value
 // garbage collection scan of a range.
+// TODO(tschottdorf): can avoid an extra message unless we're planning
+// to add more content.
 message GCMetadata {
   // The last GC scan timestamp in nanoseconds since the Unix epoch.
   optional int64 last_scan_nanos = 1 [(gogoproto.nullable) = false];
-  // The oldest unresolved write intent in nanoseconds since epoch.
-  // Null if there are no unresolved write intents.
-  optional int64 oldest_intent_nanos = 2;
 }
 
 // SequenceCacheEntry holds information which together with the key at which