[3780] MySQL and Postgres lease managers now exit on fatal error detection

[3780] MySQL and Postgres lease managers now exit on fatal error detection

src/lib/dhcpsrv/dhcpsrv_messages.mes
    added messages DHCPSRV_MYSQL_FATAL_ERROR, DHCPSRV_PGSQL_FATAL_ERROR

src/lib/dhcpsrv/mysql_lease_mgr.cc
    added MySQL client error code include

    MySqlLeaseMgr::checkError() - method is no longer inlined in
    the header.  Expanded to detect unrecoverable errors, log
    them and call exit().

src/lib/dhcpsrv/mysql_lease_mgr.h
    Removed inline implemenation of MySqlLeaseMgr::checkError(),
    and expanded commentary

src/lib/dhcpsrv/pgsql_lease_mgr.cc
    PgSqlLeaseMgr::addLeaseCommon() - now uses checkStatementError()

    PgSqlLeaseMgr::checkStatementError() - Expanded to detect
    unrecoverable errors, log them and call exit().

src/lib/dhcpsrv/pgsql_lease_mgr.h
    Expanded commentary for PgSqlLeaseMgr::checkStatementError()