cursors. This should fix Bug#11813 when InnoDB part is in 
(tested with a draft patch).
The idea of the patch is that if a storage engine supports
consistent read views, we open one when open a cursor,
set is as the active view when fetch from the cursor, and close
together with cursor close.


sql/examples/ha_archive.cc:
  - extend handlerton with cursors methods; fix coding style
sql/examples/ha_example.cc:
  - extend handlerton with cursors methods; fix coding style
sql/examples/ha_tina.cc:
  - extend handlerton with cursors methods; fix coding style
sql/ha_berkeley.cc:
  - extend handlerton with cursors methods
sql/ha_blackhole.cc:
  - extend handlerton with cursors methods; fix coding style
sql/ha_federated.cc:
  - extend handlerton with cursors methods; fix coding style
sql/ha_heap.cc:
  - extend handlerton with cursors methods; fix coding style
sql/ha_innodb.cc:
  - extend handlerton with cursors methods
sql/ha_myisam.cc:
  - extend handlerton with cursors methods; fix coding style
sql/ha_myisammrg.cc:
  - extend handlerton with cursors methods; fix coding style
sql/ha_ndbcluster.cc:
  - extend handlerton with cursors methods
sql/handler.h:
  - extend handlerton with cursors methods
sql/sql_select.cc:
  - create a consistent read view when we open a cursor,
    set it for a fetch, and free when we closing the cursor.
sql/sql_select.h:
  - add Cursor::ht_info to remember read views used in a cursor.
tests/mysql_client_test.c:
  Disable an assert that will be no longer valid when consistent
  read views in InnoDB are used.