of breaking serializability.  (Bug #3300)

ha_innobase::unlock_row(): reset the "did semi consistent read" flag

ha_innobase::was_semi_consistent_read(),
ha_innobase::try_semi_consistent_read(): new methods

row_prebuilt_t, row_create_prebuilt(): add field row_read_type for
keeping track of semi-consistent reads

row_vers_build_for_semi_consistent_read(),
row_sel_build_committed_vers_for_mysql(): new functions

row_search_for_mysql(): implement semi-consistent reads