Add a logs field in REST API returned dbgmsgs.

* These trace messages will help debugging Gerrit server side bug
  from find-owners client request.
* logs are included in every OwnersDb object but returned to
  client only for requests with debug=1 parameter.
* Add a 'nocache' parameter to compute OwnersDb without using the cache.

Change-Id: I1a62e2cf2a485aeb194e2aaec6bd546d307f32c8