[#1396] Implement CB connection recovery

[#1396] Implement CB connection recovery

src/bin/dhcp4/ctrl_dhcp4_srv.cc
    ControlledDhcpv4Srv::dbReconnect() - added logic to
    reopen CB backends (if any configured)

src/lib/database/database_connection.*
    Added DbConnectionUnusable exception
    DatabaseConnection - added:
        unusable_
        marUnusable()
        checkUnusable()

src/lib/mysql/mysql_connection.*
    MySqlConnection::checkError() - calls markUnusable()

    Added invocations of DatabaseConnection::checkUnusable()
    to functions using the connection

src/lib/pgsql/pgsql_connection.cc
    PgSqlConnection::checkStatementError() - calls markUnusable()