* based on RANGE pruning by COLUMNS (sys_trx_end) condition
* removed DEFAULT; AS OF NOW is always last; current VERSIONING as last non-empty (or first empty)
* Min/Max stats in TABLE_SHARE
* ALTER TABLE ADD PARTITION adds before AS OF NOW partition

* Syntax sugar: query-global QUERY FOR SYSTEM_TIME

[fixes #105]

* one `AS OF NOW`, multiple `VERSIONING` partitions;
* rotation of `VERSIONING` partitions by record count, time period;
* rotation is multi-threaded;
* conventional subpartitions as bottom level for versioned partitions;
* `DEFAULT` keyword selects first `VERSIONING` partition;
* ALTER TABLE ADD/DROP partition;
* REBUILD PARTITION basic operation.

[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.