Diagnostics_area refactoring for user defined exceptions

Currently members to store sqlstate, sql errno and sql condition level present in the following classes:

• Sql_condition (m_returned_sqlstate, m_sql_errno, m_level)
• Diagnostics_area (m_sql_errno, m_sqlstate)
• Sql_condition_info (sql_errno, sql_state, level)
• sp_condition_value (mysqlerr, sql_state)

This produces a lot of duplicate code.
For the purposes of MDEV-10587 we'll also have to add a pointer to user defined exceptions at least in the following classes:

• Sql_condition
• Sql_condition_info

That will generate more duplicate code.
To avoid code duplication, we'll introduce the following class hierarchy:

Sql_state

  Sql_state_errno

    Diagnostics_area

    sp_condition_value

    Sql_state_errno_level

      Sql_condition_info

      Sql_condition