Ensure rewrite in vlog is within transactional limits (#911)

* Ensure rewrite in vlog is within transactional limits

With this commit the temporary list of entries built during value log
iteration is committed before it overflows transactional limits
(maxBatchSize and maxBatchCount).

Fixes #907

value.go

@@ -397,16 +397,19 @@ func (vlog *valueLog) rewrite(f *logFile, tr trace.Trace) error {
 			}
 
 			ne.Value = append([]byte{}, e.Value...)
-			wb = append(wb, ne)
-			size += int64(e.estimateSize(vlog.opt.ValueThreshold))
-			if size >= 64*mi {
+			es := int64(ne.estimateSize(vlog.opt.ValueThreshold))
+			// Ensure length and size of wb is within transaction limits.
+			if int64(len(wb)+1) > vlog.opt.maxBatchCount ||
+				size+es > vlog.opt.maxBatchSize {
 				tr.LazyPrintf("request has %d entries, size %d", len(wb), size)
 				if err := vlog.db.batchSet(wb); err != nil {
 					return err
 				}
 				size = 0
 				wb = wb[:0]
 			}
+			wb = append(wb, ne)
+			size += es
 		} else {
 			vlog.db.opt.Warningf("This entry should have been caught. %+v\n", e)
 		}