because the statement is TRUNCATE, not DELETE

* again, as in 10.2, NOW is a keyword only if followed by parentheses
* use AS OF CURRENT_TIMESTAMP or AS OF NOW()
* AS OF CURRENT_TIMESTAMP and AS OF NOW() mean AS OF NOW(6),
  not AS OF NOW(0), (same behavior as in a DEFAULT clause)

Do not generate fake values when adding an auto-inc column to a versioned
table. This is not a auto-inc issue, but a more general case of adding
a not nullalble unique column to a table with history. We don't support
it yet, not even with a special auto-inc hack. As a workaround, one
can use a nullable unique column, that works.

use num as the rest of the parser

This reverts commit 1cfdff5fe3c0044fa08adf5da9088dc46cc646db.

Fix it differently.
Cleanup, test results didn't change.

because it'll fail to start without it

This is a separate feature, unrelated to system versioning

We may keep it or not, but either way, it's a separate discussion,
if we keep it, it won't be in the system versioning branch

SELECT * FROM t1 FOR SYSTEM_TIME AS OF ...

becomes ambiguous, but it's the same as with

SELECT ... UNION SELECT ... ORDER BY ...

Applicable to vanilla version.
Applies to Item_cond_and, etc.: items with allocating copy_andor_structure().

Tests affected (forced mode):

main.subselect_sj
main.subselect_sj_jcl6

Tests affected (forced mode):

main.ps \
main.user_var \
main.myisam_explain_non_select_all \
main.opt_tvc \
main.subselect \
main.subselect_no_exists_to_in \
main.derived \
main.derived_opt \
main.update

MTR_COMBINATIONS contains comma-separated list of currently applied
combinations "x,y,...". MTR_COMBINATION_X, MTR_COMBINATION_Y, ... are
set to 1 for this list of combinations.

Fixed dead check: non-existent element $tinfo->{combination}.

Better use this clause:

use warnings FATAL => 'uninitialized';

But this must be very well tested.

Tests affected (forced mode):

main.type_timestamp

Tests affected (forced versioning):

rpl.rpl_row_merge_engine

Affected tests (forced mode):

main.index_merge_myisam

After optimization versioning AND takes operands from WHERE AND
leaving `select_lex->where` as empty dangling AND.

Affected tests (forced mode):

main.win
main.selectivity_innodb
main.subselect_cache
main.analyze_stmt_orderby

Related to #226 (909867d0).

Affected tests (forced mode): rpl.rpl_timezone

Affected tests (forced mode): binlog_encryption.encrypted_slave

Affected tests (forced mode): binlog_encryption.encrypted_slave

row_prebuilt_t::versioned_write: Cache the value of TABLE::versioned_write()
in ha_innobase::build_template()