join cache module.
Without these calls SELECTs over tables with virtual columns
that used join cache could return wrong results. This could
be seen with the test case added into vcol_misc.test 

- The problem was that DuplicateWeedout strategy setup code wasn't aware of the 
  fact that join buffering will be used and applied optimization that doesn't work
  together with join buffering. Fixed by making DuplicateWeedout setup code to have 
  a pessimistic check about whether there is a chance that join buffering will be 
  used.
- Make JOIN_CACHE_BKA::init() correctly process Copy_field elements that denote saving
  current rowids in the join buffer.

mysql-test/r/subselect_sj2.result:
  Update test results
mysql-test/r/subselect_sj2_jcl6.result:
  Update test results
mysql-test/r/subselect_sj_jcl6.result:
  Testcase
mysql-test/t/subselect_sj2.test:
  Update test results
mysql-test/t/subselect_sj_jcl6.test:
  Testcase
sql/opt_subselect.cc:
  - The problem was that DuplicateWeedout strategy setup code wasn't aware of the 
    fact that join buffering will be used and applied optimization that doesn't work
    together with join buffering. Fixed by making DuplicateWeedout setup code to have 
    a pessimistic check about whether there is a chance that join buffering will be 
    used.
sql/sql_join_cache.cc:
  Make JOIN_CACHE_BKA::init() correctly process Copy_field elements that denote saving current rowids in the join buffer.
sql/sql_select.cc:
  Added a question note

The function JOIN_CACHE::read_all_record_fields could return 0
for an incremental join cache in two cases:
1. there were no more records in the associated join buffer
2. there was no table fields stored in the join buffer.
As a result the function JOIN_CACHE::get_record() could
return prematurely and did not read all needed fields from
join buffers into the record buffer.

Now the function JOIN_CACHE::read_all_record_fields returns
-1 if there are no more records in the associated join buffer.

Made sure that join buffers could be used for inner tables of
any semi-join when the first match strategy is employed.

- Factor out subquery code into sql/opt_subselect.{h,cc}
- Stop using the term "confluent" (was used due to misreading the dictionary)

- Enable semi-join handling in the join cache code

WL#2771 "Block Nested Loop Join and Batched Key Access Join"