Make lots of fixes to handle the most complex case of reorganising
of partitions where two-phased processes are needed in some cases.


sql/ha_partition.cc:
  Rewrote the rename partitions and drop partitions to align with how
  the table log handles things.
sql/handler.h:
  Added new entry to partition_element to keep track of log entry for
  a partition during ALTER TABLE that reorganises existing partitions.
sql/mysql_priv.h:
  Converted 'd', 'e' and so forth to constants with somewhat more
  descriptive names
  Added method to inactivate log entries
sql/sql_partition.cc:
  Fix change of partitions
sql/sql_table.cc:
  More constants with somewhat descriptive names
  Moved around some methods between internal part and external part
  Added new method to handle inactivation of log entries