Document store: add JSON type and codec.
The JSON binary representation is same with MySQL 5.7. we prefer
this not bson because 1) bson only supports JSON compound types
but not JSON primitive types, and 2) this representation is better
than bson on random access.
This PR now can support these statements:
```
CREATE TABLE t (a json_field);
INSERT INTO t (a) values ('{"a": "b"}');
SELECT * FROM t;
```
JSON codec uses MySQL 5.7 compatible format, which doesn't support use JSON field as key or index. We will limit this in tidb later.
1e4bf4775c