history.  This addresses Mantis issue #116.

dict_index_t: Enable the storage of trx_id.

row_prebuilt_t: Make many fields bit-fields to reduce the memory
footprint. Add index_usable.

ha_innobase::change_active_index(): Check if the index is usable and
set prebuilt->index_usable accordingly. Unfortunately, the return
status of this function is ignored by MySQL, and the actual refusal to
use the index must be made in row_search_for_mysql().

row_search_for_mysql(): Return DB_MISSING_HISTORY if
!prebuilt->index_usable.

convert_error_code_to_mysql(): Map DB_MISSING_HISTORY to
HA_ERR_TABLE_DEF_CHANGED.

innodb-index.test: Add a test case where access to a newly created
secondary index must be blocked for old transactions.

rb://100 approved by Heikki Tuuri