Add sqlite backend

[BrendanBall]

This change is 

[dennwc]

The code looks good, I think we can merge it right away. I only want to run tests locally to make sure everything is in order.

Also, can you please update README.md, ./docs/Configuration.md and any other places that list supported backends (just search for postgres in the code)?

Reviewed 8 of 8 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved

[BrendanBall]

Thanks for the awesome response time :D . I've just pushed some docs. Let me know if you'd like to add anything else.

[dennwc]

Thanks for adding the documentation! :) Also make sure to add yourself to AUTHORS and CONTRIBUTORS.

All regular tests pass (:tada:), but I've hit an error when running the integration test suite (RUN_INTEGRATION=true go test ./...). Haven't checked yet what the reason is, though.

[BrendanBall]

Cool, I'll check on that tomorrow

[BrendanBall]

So here are the failing tests:

➜  cayley git:(master) ✗ RUN_INTEGRATION=true go test ./graph/sql/sqlite -timeout 30m
--- FAIL: TestSqlite (0.00s)
    --- FAIL: TestSqlite/qs (1020.98s)
        --- FAIL: TestSqlite/qs/integration (1020.88s)
            integration.go:490: loaded data in 18.9748405s
            --- FAIL: TestSqlite/qs/integration/three_huge_sets_with_small_intersection (300.00s)
                integration.go:523: Error: interrupted
                integration.go:533: 5m0.000410593s three huge sets with small intersection
                integration.go:536: Unexpected number of results, got:0 expect:2 on three huge sets with small intersection.
            --- FAIL: TestSqlite/qs/integration/Keanu_with_other_in_The_Net (300.00s)
                integration.go:533: 5m0.000397876s Keanu with other in The Net
                integration.go:536: Unexpected number of results, got:0 expect:2 on Keanu with other in The Net.
            --- FAIL: TestSqlite/qs/integration/Keanu_and_Bullock_with_other (300.00s)
                integration.go:533: 5m0.000344452s Keanu and Bullock with other
                integration.go:536: Unexpected number of results, got:6 expect:166 on Keanu and Bullock with other.
FAIL
FAIL	github.com/cayleygraph/cayley/graph/sql/sqlite	1021.010s

No idea why those 3 tests are failing. Each of those take 5 min to run so it's pretty difficult to debug. Might take a while to figure out.

EDIT: So I've found that the query timeout for the tests is set to 5 min, which is why all 3 these tests fail at 5 min. I'm not sure how long the other backends take to run these tests, but either sqlite is insanely slow for this case or these test queries are pretty crazy.

[dennwc]

@BrendanBall That might explain why they fail for the same SQL implementation that works for PG and MySQL.

I was confused by Unexpected number of results, got:0 expect:2 without any errors, but that's probably a problem with context error propagation in iterators.

If setting a higher timeout helps, I will consider this solved.

Sure, we might need some optimizations for SQLite to be faster for those kinds of queries, but it's worth merging in the current state as well.

[BrendanBall]

the test TestSqlite/qs/integration/three_huge_sets_with_small_intersection didn't even complete within an hour :/ . So much for quickly adding a sqlite backend :P

[dennwc]

OK, I would say that the fact that backend is slow on integration test shouldn't block this PR. We can make some optimizations later, if necessary.

I will check the PR again and merge it if there are no objections.