using prepared statements.


sql/field.cc:
  Using MAX_DATETIME_WIDTH or MAX_DATETIME_COMPRESSED_WIDTH
  constants for the length of DATETIME fields.
  
  Using MAX_DATE_STRING_REP_LENGTH for allocating buffers
  for date/time/... string representation.
sql/item_timefunc.cc:
  Using MAX_DATETIME_WIDTH or MAX_DATETIME_COMPRESSED_WIDTH
  constants for the length of DATETIME fields.
  
  Using MAX_DATE_STRING_REP_LENGTH for allocating buffers
  for date/time/... string representation.
sql/unireg.h:
  Introduce a constant for length of datetime compressed
  format (YYYYMMDDHHMMSS).