v3: support for aggregation and sorting of TEXT

[sougou]

This change adds support for aggregating and sorting of columns
that may need to follow collation rules.

The feature depends on mysql's weight_string function
that returns a lexically comparable value of any text column.
If a group by or order by uses a text column like varchar, then
v3 will additionally request the weight_string versions of such
columns and perform ordering and aggregation using those values
instead.

The identification of a text column is currently based on a new
"columns" field in the vschema that allows you to specify the
type of each column. This part was implemented in a previous PR.

Two approaches were possible:

1. Request additional weight_string values at the time of push-down.
2. Rewire the primitives to request and use the additional weight_string
   values during the Wireup phase.
   I went with option 2 because it minimizes overall impact of the
   code, which will allow us to yank this behavior out when we
   implement collation aware sorting in vtgate. Also, since all
   the weight_string columns get added at the end, we only have
   to truncate the rows before returning what's needed.

The following changes were made:

• sqltypes: result truncate functionality.
• engine: Add TruncateColumnCount field that can be used to truncate
  a result if needed.
• planbuilder: Change Wireup to check and request weight_strings.
  If weight_string was requested, set the row truncation to make
  sure that the weight_string values don't get passed on.

[dumbunny]

LGTM

[demmer]

Some minor comments but overall this LGTM.

Overall I would really love if we could find an alternative to weight_string but as discussed on Slack that doesn't look to be the case given the golang utf-8 limitations.

[demmer]

Can you rephrase this as "truncated to the specified number of columns".
I was initially confused thinking that this would truncate the individual fields.

[sougou]

Done.

[demmer]

It seems cleaner and I suspect more efficient on the gc to only create weightStrings when needed rather than always create the empty array.

[sougou]

Unfortunately, this is a map. There's no autocreation support for it like slices.
Deferring the creation will introduce conditionals in multiple places (nil-check->make).

Besides, this is planbuilder time, which should be low QPS ideally.

[demmer]

I assume removing these is intentional (which I support) but that seems orthogonal to these changes.

[sougou]

Yeah. There was too much spam in tests.

[indera-shsp]

@sougou Is this a known bug?

select id, companyName As test from companyProfile where id = 999123 order by companyName;
+-------+-------------+
| id    | test        |
+-------+-------------+
|999123 | TheCompany  |
+-------+-------------+
1 row in set (0.09 sec)


select id, companyName As test from companyProfile where id in(999123) order by companyName;


ERROR 1064 (42000): vtgate: http://vtgate-...: target: app.-80.master, used tablet: staging-978852100 (app-x-80x-
replica-0.vttablet.default.svc.cluster.local):
 vttablet: 
rpc error: code = InvalidArgument desc = You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'test) from companyProfile where 1 != 1' at line 1 (errno 1064) (sqlstate 42000) (CallerID: ...): Sql: "selec