Added ChangeLog entry

src/bin/dhcp4/ctrl_dhcp4_srv.*
    ControlledDhcpv4Srv::dbLostCallback() - schedules a shutdown
    once retries have been exhausted/disableld

src/bin/dhcp6/ctrl_dhcp6_srv.*
    ControlledDhcpv6Srv::dbLostCallback() - schedules a shutdown
    once retries have been exhausted/disableld

src/lib/database/database_connection.h
    class DbUnrecoverableError - new exception

src/lib/mysql/mysql_connection.h
    MySqlConnection::check_error() - throws DbUnrecoverableError
    instead of calling exit()

src/lib/pgsql/pgsql_connection.*
    PgSqlConnection::checkStatementError() - throws DbUnrecoverableError
    instead of calling exit()