Tweak external file ingestion seqno logic under universal compaction

Summary:
Right now it is possible that a file gets assigned to L0 but also assigned the seqno from a higher level which it doesn't fit
Under the current impl, it is possibe that seqno in lower levels (Ln) can be equal to smallest seqno of higher levels (Ln-1), which is undesirable from universal compaction's point of view.
This should fix the intermittent failure of `ExternalSSTFileBasicTest.IngestFileWithGlobalSeqnoPickedSeqno`
Closes https://github.com/facebook/rocksdb/pull/3411

Differential Revision: D6813802

Pulled By: miasantreble

fbshipit-source-id: 693d0462fa94725ccfb9d8858743e6d2d9992d14

db/external_sst_file_ingestion_job.cc

@@ -473,7 +473,10 @@ Status ExternalSstFileIngestionJob::AssignLevelAndSeqnoForIngestedFile(
                             return f1->largest_seqno < f2->largest_seqno;
                           }))
                 ->largest_seqno;
-        if (level_largest_seqno != 0) {
+        // should only assign seqno to current level's largest seqno when
+        // the file fits
+        if (level_largest_seqno != 0 &&
+            IngestedFileFitInLevel(file_to_ingest, lvl)) {
           *assigned_seqno = level_largest_seqno;
         } else {
           continue;