Fix a TestGet failure when user defined timestamp is enabled (#11249)

Summary:
Stressing small DB with small number of keys and user-defined timestamp enabled usually fails pretty quickly in TestGet.

Example command to reproduce the failure:

` tools/db_crashtest.py blackbox --enable_ts --simple --delrangepercent=0 --delpercent=5 --max_key=100 --interval=3 --write_buffer_size=262144 --target_file_size_base=262144 --max_bytes_for_level_base=262144 --subcompactions=1`

Example failure: `error : inconsistent values for key 0000000000000009000000000000000A7878: expected state has the key, Get() returns NotFound.`

Fixes this test failure by refreshing the read up to timestamp to the most up to date timestamp, a.k.a now, after a key is locked.  Without this, things could happen in this order and cause a test failure:

<table>
  <tr>
    <th>TestGet thread</th>
    <th> A writing thread</th>
  </tr>
  <tr>
    <td>read_opts.timestamp = GetNow()</td>
    <td></td>
  </tr>
  <tr>
    <td></td>
    <td>Lock key, do write</td>
  </tr>
  <tr>
    <td>Lock key, read(read_opts) return NotFound</td>
    <td></td>
  </tr>
</table>

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11249

Reviewed By: ltamasi

Differential Revision: D43551302

Pulled By: jowlyzhang

fbshipit-source-id: 26877ab379bdb97acd2682a2632bc29718427f38

db_stress_tool/no_batched_ops_stress.cc

@@ -492,6 +492,11 @@ class NonBatchedOpsStressTest : public StressTest {
     ReadOptions read_opts_copy = read_opts;
     std::string read_ts_str;
     Slice read_ts_slice;
+    if (FLAGS_user_timestamp_size > 0) {
+      read_ts_str = GetNowNanos();
+      read_ts_slice = read_ts_str;
+      read_opts_copy.timestamp = &read_ts_slice;
+    }
     bool read_older_ts = MaybeUseOlderTimestampForPointLookup(
         thread, read_ts_str, read_ts_slice, read_opts_copy);
 
@@ -514,7 +519,7 @@ class NonBatchedOpsStressTest : public StressTest {
       // found case
       thread->stats.AddGets(1, 1);
       // we only have the latest expected state
-      if (!FLAGS_skip_verifydb && !read_opts_copy.timestamp &&
+      if (!FLAGS_skip_verifydb && !read_older_ts &&
           thread->shared->Get(rand_column_families[0], rand_keys[0]) ==
               SharedState::DELETION_SENTINEL) {
         thread->shared->SetVerificationFailure();