Closes #73

IB:
* removed CONCURR_TRX from VTQ;
* new fields in VTQ: COMMIT_ID, ISO_LEVEL.

SQL:
* renamed BEGIN_TS, COMMIT_TS to VTQ_BEGIN_TS, VTQ_COMMIT_TS;
* new functions: VTQ_COMMIT_ID, VTQ_ISO_LEVEL, VTQ_TRX_ID, VTQ_TRX_SEES, VTQ_TRX_SEES_EQ;
* versioned SELECT for IB uses VTQ_TRX_SEES, VTQ_TRX_SEES_EQ.

Closes #71

Closes #72

Syntax extension: TIMESTAMP/TRANSACTION keyword can be used before FROM ... TO, BETWEEN ... AND.
Example:
    SELECT * FROM t1 FOR SYSTEM_TIME TIMESTAMP FROM '1-1-1' TO NOW();

Closes #27

Fixes #53

Fixes #52

Run `mtr --suite=versioning` or `mtr versioning.<test-name>`

create: 5de65ef52df9f941ec8eea93f4801283e3d7fa05
select_pkeycache, select_jcl6: ded545be9f7a416d1d50d653ab278dd0fc7e6c5e

ded545be9f7a416d1d50d653ab278dd0fc7e6c5e

Moved opt_for_system_time_clause from table identifier (which binds
it to single table) to table expression.

* BEGIN_TS(), COMMIT_TS() SQL functions;
* VTQ instead of packed stores secs + usecs like my_timestamp_to_binary() does;
* versioned SELECT to IB is translated with COMMIT_TS();
* SQL fixes:
  - FOR_SYSTEM_TIME_UNSPECIFIED condition compares to TIMESTAMP_MAX_VALUE;
  - segfault fix #36: multiple execute of prepared stmt;
  - different tables to same stored procedure fix (#39)
* Fixes of previous parts: ON DUPLICATE KEY, other misc fixes.

These features are defined only in XtraDB:
  chaged_page_bitmaps,
  innodb_changed_pages