Column Compression Extensions for 5.7

https://github.com/percona/percona-server/pull/1147

Add support for compressed JSON columns.

Add support for indexes on virtual columns based on compressed fields:
- Purge threads now also need to have access to prebuilt object.

Add support for COMPRESSED attribute in STORED generated columns.
It should be possible to make generated columns compressed with optional compression dictionary. Please, notice that this makes sense only for STORED generated columns, for VIRTUAL ones the ER_UNSUPPORTED_COMPRESSED_COLUMN_TYPE error must be reported.

New generated column definition syntax:
col_name data_type [GENERATED ALWAYS] AS (expression)
  [VIRTUAL | STORED] [UNIQUE [KEY]] [COMMENT comment]
  [[NOT] NULL] [[PRIMARY] KEY]
  [COLUMN_FORMAT {DEFAULT|COMPRESSED} [WITH COMPRESSION_DICTIONARY dict_name]]

The new syntax is allowed in both 'CREATE TABLE' and 'ALTER TABLE' statements. The latter can also be used to apply/remove COMPRESSED attribute to/from an existing STORED generated column.
For instance
ALTER TABLE t MODIFY g BLOB GENERATED ALWAYS AS (a) STORED COLUMN_FORMAT COMPRESSED
or
ALTER TABLE t MODIFY g BLOB GENERATED ALWAYS AS (a) STORED COLUMN_FORMAT DEFAULT