Work-in-progress.

Tests part I.

Improvements on this cset:
- moved rpl_row_idempotency contents to a file in extra/rpl_tests
- made rpl_row_idempotency run twice: one time with TABLE_SCAN
  and another with HASH_SCAN by including 
  extra/rpl_tests/rpl_row_idempotency.test
- added rpl_row_hash_scan.test with several assertions on
  HASH_SCAN algorithm (also provides coverage the new 
  code introduced).
- fixed slave_rows_search_algorithm_basic.test that was broken
  from previous commit.
- instrumented log_event.cc, mysqld.cc and mysqld.h with a new
  status var: slave_rows_last_search_algorithm_used so that 
  some sanity tests are written.