Summary:
This PR fixes the issue of unstable snapshot during external SST file ingestion. Credit ajkr for the following walk through:  consider these relevant steps for of IngestExternalFile():

(1) increase seqno while holding mutex -- https://github.com/facebook/rocksdb/blob/677d2b4a8f8fd19d0c39a9ee8f648742e610688d/db/db_impl/db_impl.cc#L4768
(2) LogAndApply() -- https://github.com/facebook/rocksdb/blob/677d2b4a8f8fd19d0c39a9ee8f648742e610688d/db/db_impl/db_impl.cc#L4797-L4798
  (a) write to MANIFEST with mutex released https://github.com/facebook/rocksdb/blob/a96a4a2f7ba7633ab2cc51defd1e923e20d239a6/db/version_set.cc#L4407
  (b) apply to in-memory state with mutex held

A snapshot taken during (2a) will be unstable. In particular, queries against that snapshot will not include data from the ingested file before (2b), and will include data from the ingested file after (2b).

Pull Request resolved: https://github.com/facebook/rocksdb/pull/10051

Test Plan:
Added a new unit test: `ExternalSSTFileBasicTest.WriteAfterReopenStableSnapshotWhileLoggingToManifest`.
```
make external_sst_file_basic_test
./external_sst_file_basic_test
```

Reviewed By: ajkr

Differential Revision: D36654033

Pulled By: cbi42

fbshipit-source-id: bf720cca313e0cf211585960f3aff04853a31b96