* New syntax has been added to output EXPLAIN in a structured form: EXPLAIN FORMAT=JSON

* Traditional EXPLAIN output format has been modified:
  1. The "Extra" column:
     * metadata change:
       was not-nullable,
       from now it is nullable;
     * default value:
       was: empty string,
       from now: NULL
  2. The "ref" column:
     * default value:
       was: mix of empty string in one cases and NULL in others,
       from now: NULL
  3. The "partitions" column:
     * default value:
       was: empty string,
       from now: NULL

* New system variable has been added: end_markers_in_json

* The old optimizer_trace variable has been modified to not accept the "end_marker" parameter (moved to "end_markers_in_json").

* New error message has been added: ER_UNKNOWN_EXPLAIN_FORMAT

* mysqltest and MTR utilities have updated to accept a new command line parameter: --json-explain-protocol