metamorphic: reenable downloadOp

Reenable downloadOp and add code to fail the test if any single op
runs for too long. Currently the timeout is very conservative to
prevent flakiness in stress and/or race tests (but it can be turned
down manually to investigate a specific issue).

Author: RaduBerinde

metamorphic/config.go

@@ -143,7 +143,7 @@ func DefaultOpConfig() OpConfig {
 			OpBatchCommit:                 5,
 			OpDBCheckpoint:                1,
 			OpDBCompact:                   1,
-			OpDBDownload:                  0, // TODO(radu, bilal): fix #3430 and reenable.
+			OpDBDownload:                  1,
 			OpDBFlush:                     2,
 			OpDBRatchetFormatMajorVersion: 1,
 			OpDBRestart:                   2,