Coverity scan issues

I triaged new issues detected by Coverity scan and below ones could be checked more carefully:

• High: time_t casting to uint_32 - assuming we are doing static_cast, we should be ok until year 2106. One cases misses the static_cast.
  • https://scan5.scan.coverity.com/reports.htm#v60426/p10289/fileInstanceId=250303669&defectInstanceId=53277773&mergedDefectId=1533185
• Medium: dereferencing invalid iterator - in most cases it seems to be false positive, but there is one place where it could be checked - this could have separate ticket
  • https://scan5.scan.coverity.com/reports.htm#v60426/p10289/fileInstanceId=250303968&defectInstanceId=53270168&mergedDefectId=1533337
• Low: used auto causing copy - this is low priority, but could be fixed anyway, just const auto& could be used.
  • https://scan5.scan.coverity.com/reports.htm#v60426/p10289/fileInstanceId=250303713&defectInstanceId=53363199&mergedDefectId=1539484
  • https://scan5.scan.coverity.com/reports.htm#v60426/p10289/fileInstanceId=250303704&defectInstanceId=53363142&mergedDefectId=1539483
  • https://scan5.scan.coverity.com/reports.htm#v60426/p10289/fileInstanceId=250303706&defectInstanceId=53363182&mergedDefectId=1539482
  • https://scan5.scan.coverity.com/reports.htm#v60426/p10289/fileInstanceId=250304018&defectInstanceId=53270328&mergedDefectId=1532980 and all others in src/hooks/dhcp/lease_cmds/lease_cmds.cc
  • https://scan5.scan.coverity.com/reports.htm#v60426/p10289/fileInstanceId=250304037&defectInstanceId=53270311&mergedDefectId=1532961 and all others in src/hooks/dhcp/mysql_cb/mysql_cb_dhcp*.cc
  • https://scan5.scan.coverity.com/reports.htm#v60426/p10289/fileInstanceId=250304057&defectInstanceId=53270884&mergedDefectId=1532978 and all others in src/hooks/dhcp/pgsql_cb/pgsql_cb_dhcp*.cc