add support for sqlite RETURNING clause

[elv-gilles]

This PR adds support for sqlite RETURNING clause.

Requests involving the returning clause must include a boolean field before the SQL statement (see ParseRequest in http/request_parser.go).

[otoolep]

Thanks for your contribution, however this is not the way to solve this. Instead the SQL parser that rqlite uses should be updated to detect when a SQL statement contains RETURNING and then rqlite should automatically do the right thing. That is my plan anyway.

The SQL parser is here: https://github.com/rqlite/sql

It would that code that needs support added for detecting RETURNING.

[otoolep]

Unless I am missing something that will prevent RETURNING support being added to the SQL parser, I do not plan to merge this PR.

[otoolep]

Thanks again for your contribution. If you'd instead like to add support to the SQL parser Go code, I would be interested in a such a PR. Basically some way that the parser could tell you RETURNING was in a given statement, allowing new logic to kick in in rqlite.

[otoolep]

I would suggest you propose some of your ideas first though (usually as a GitHub issue), as I wouldn't like folks to spend a long time coding something, only to find out later that it didn't make sense when created as a PR.

[elv-gilles]

OK, no problem: I missed the SQL parser.

I agree this is a better solution and am looking into the parser.

[elv-gilles]

I implemented support in the sql parser (rqlite/sql#9), which unfortunately seems to have an issue (rqlite/sql#10).

For the time being, I'll use what I proposed in this PR (from fork)

[elv-gilles]

Notes:

• relates to Support for RETURNING in api response #1157
• I tried several golang parsers (including rqlite/sql) and found github.com/auxten/postgresql-parser supporting complex statements and the returning clause.