network when a query was slow.

When a query is slow, sent a special flag to the client
indicating this fact.

Add a test case.
Implement review comments.



include/mysql_com.h:
  Clear SERVER_QUERY_WAS_SLOW at end of each statement.
  Since this patch removes the technique when 
  thd->server_status is modified briefly only to
  execute my_eof(), reset more server status
  bit that may remain in the status from
  execution of the previous statement.
sql/protocol.cc:
  Always use thd->server_status to 
  in net_* functions to send the latest
  status to the client.
sql/sp_head.cc:
  Calculate if a query was slow before
  sending EOF packet.
sql/sql_cursor.cc:
  Remove juggling with thd->server_status.
  The extra status bits are reset at
  start of the next statement.
sql/sql_db.cc:
  Remove juggling with thd->server_status.
  The extra status bits are reset at
  start of the next statement.
sql/sql_error.cc:
  Remove m_server_status member,
  it's not really part of the Diagnostics_area.
sql/sql_error.h:
  Remove server_status member, it's
  not part of the Diagnostics_area.
  The associated hack is removed as well.
sql/sql_parse.cc:
  Do not calculate if a query was
  slow twice. Use a status flag in thd->server_status.
tests/mysql_client_test.c:
  Add a test case for Bug#57058.
  Check that the status is present
  at the client, when sent.