[fixes #88]

* sys fields are NULL by default (with exceptions, see comment about NOT_NULL_FLAG in #77);
* error codes renamed, messages cleared out;
* SHOW CREATE TABLE fixed;
* set_max() fix;
* redundant flag setters/getters removed;
* flags are set in sql_yacc.yy, redundant copy_info_about_generated_fields() eliminated.

WARN_MODE=early
print warnings at compilation stage

WARN_MODE=late
print warnings at build finish stage

WARN_MODE=both
print both at early and at later stages

Replaced codes:

ER_NO_VERSIONED_FIELDS_IN_VERSIONED_TABLE
ER_MISSING_WITH_SYSTEM_VERSIONING
ER_SYS_START_NOT_SPECIFIED
ER_SYS_END_NOT_SPECIFIED
ER_MISSING_PERIOD_FOR_SYSTEM_TIME
ER_PERIOD_FOR_SYSTEM_TIME_CONTAINS_WRONG_START_COLUMN
ER_PERIOD_FOR_SYSTEM_TIME_CONTAINS_WRONG_END_COLUMN
ER_SYS_START_AND_SYS_END_SAME
ER_SYS_START_MORE_THAN_ONCE
ER_SYS_END_MORE_THAN_ONCE

with:

ER_VERS_WRONG_PARAMS
ER_VERS_FIELD_WRONG_TYPE

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