WritePrepared Txn: update compaction_iterator_test and db_iterator_test

Summary:
Update compaction_iterator_test with write-prepared transaction DB related tests. Transaction related tests are group in CompactionIteratorWithSnapshotCheckerTest. The existing test are duplicated to make them also test with dummy SnapshotChecker that will say every key is visible to every snapshot (this is okay, we still compare sequence number to verify visibility). Merge related tests are disabled and will be revisit in another PR.

Existing db_iterator_tests are also duplicated to test with dummy read_callback that will say every key is committed.
Closes https://github.com/facebook/rocksdb/pull/3466

Differential Revision: D6909253

Pulled By: yiwu-arbug

fbshipit-source-id: 2ae4656b843a55e2e9ff8beecf21f2832f96cd25

db/compaction_iterator.cc

@@ -537,6 +537,11 @@ void CompactionIterator::NextFromInput() {
       //
       // Note:  Dropping this Delete will not affect TransactionDB
       // write-conflict checking since it is earlier than any snapshot.
+      //
+      // It seems that we can also drop deletion later than earliest snapshot
+      // given that:
+      // (1) The deletion is earlier than earliest_write_conflict_snapshot, and
+      // (2) No value exist earlier than the deletion.
       ++iter_stats_.num_record_drop_obsolete;
       if (!bottommost_level_) {
         ++iter_stats_.num_optimized_del_drop_obsolete;
@@ -621,9 +626,12 @@ void CompactionIterator::PrepareOutput() {
   // and the earliest snapshot is larger than this seqno
   // and the userkey differs from the last userkey in compaction
   // then we can squash the seqno to zero.
-
+  //
   // This is safe for TransactionDB write-conflict checking since transactions
   // only care about sequence number larger than any active snapshots.
+  //
+  // Can we do the same for levels above bottom level as long as
+  // KeyNotExistsBeyondOutputLevel() return true?
   if ((compaction_ != nullptr &&
       !compaction_->allow_ingest_behind()) &&
       ikeyNotNeededForIncrementalSnapshot() &&