MDEV-5800 InnoDB support for indexed vcols

  * remove old 5.2+ InnoDB support for virtual columns
  * enable corresponding parts of the innodb-5.7 sources
  * copy corresponding test cases from 5.7
  * copy detailed Alter_inplace_info::HA_ALTER_FLAGS flags from 5.7
     - and more detailed detection of changes in fill_alter_inplace_info()
  * more "innodb compatibility hooks" in sql_class.cc to
     - create/destroy/reset a THD (used by background purge threads)
     - find a prelocked table by name
     - open a table (from a background purge thread)

  * different from 5.7:
    - new service thread "thd_destructor_proxy" to make sure all THDs are
      destroyed at the correct point in time during the server shutdown
    - proper opening/closing of tables for vcol evaluations in
       + FK checks (use already opened prelocked tables)
       + purge threads (open the table, MDLock it, add it to tdc, close
         when not needed)
    - cache open tables in vc_templ
    - avoid unnecessary allocations, reuse table->record[0] and table->s->default_values
    - not needed in 5.7, because it overcalculates:
      + tell the server to calculate vcols for an on-going inline ADD INDEX
      + calculate vcols for correct error messages

  * update other engines (mroonga/tokudb) accordingly

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