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