deadlock during oltp_point_select:

sysbench tests failed due to a deadlock during oltp_point_select. Notice that this was a single node and also that it thought it went incoherent:
```
2019/02/04 21:02:50 941 pwrites, 10179072 bytes, avg time 0ms
2019/02/04 21:02:50 LSN 2203:10632190 diff 9692370
2019/02/04 21:03:08 [ERROR] trying writelock (downgrade_noelect 140651176560384), last writelock is bdb_open_int 0
2019/02/04 21:03:08 [ERROR] THD 140651189602048:recover_deadlock, and lock desired
2019/02/04 21:03:08 [ERROR] THD 140650937706240:recover_deadlock, and lock desired
2019/02/04 21:03:08 [ERROR] THD 140651146020608:recover_deadlock, and lock desired
2019/02/04 21:03:08 [ERROR] THD 140651159262976:recover_deadlock, and lock desired
2019/02/04 21:03:08 [ERROR] THD 140651021747968:recover_deadlock, and lock desired
2019/02/04 21:03:08 [ERROR] THD 140650954499840:recover_deadlock, and lock desired
2019/02/04 21:03:08 [ERROR] THD 140650946103040:recover_deadlock, and lock desired
2019/02/04 21:03:08 [ERROR] THD 140651183400704:recover_deadlock, and lock desired
2019/02/04 21:03:08 [ERROR] THD 140650977634048:recover_deadlock, and lock desired
2019/02/04 21:03:08 [ERROR] THD 140650962896640:recover_deadlock, and lock desired
2019/02/04 21:03:08 bdb_downgrade_int line 4866 calling rep_start as client with egen 0
2019/02/04 21:03:08 [ERROR] bdb_downgrade_int returning
2019/02/04 21:03:08 [ERROR] handle_newsql_request line 2305 td 3842307840 new query on incoherent node, dropping socket
2019/02/04 21:03:08 [ERROR] handle_newsql_request line 2305 td 3882985216 new query on incoherent node, dropping socket
2019/02/04 21:03:08 [ERROR] handle_newsql_request line 2305 td 3852179200 new query on incoherent node, dropping socket
2019/02/04 21:03:08 [ERROR] handle_newsql_request line 2305 td 3858892544 new query on incoherent node, dropping socket
```

_Originally posted by @nirbhayc in https://github.com/bloomberg/comdb2/pull/1418#issuecomment-460490554_