* Fix "cannot start a transaction within a transaction" issue
[why]
If db.BeginTx(ctx, nil) context is cancelled too fast, "BEGIN" statement can be
completed inside DB, but we still try to cancel it with sqlite3_interrupt.
In such case we get context.Cancelled or context.DeadlineExceeded from exec(),
but operation really completed. Connection returned into pool, and returns "cannot
start a transaction within a transaction" error for next db.BeginTx() call.
[how]
Handle status code returned from cancelled operation.
[testing]
Added unit-test which reproduces issue.
* Reduce TestQueryRowContextCancelParallel concurrency
[why]
Tests times out in travis-ci when run with -race option.
[why]
Context cancellation goroutine is not in sync with Next() method lifetime.
It leads to sql.ErrNoRows instead of context.Canceled often (easy to reproduce).
It leads to interruption of next query executed on same connection (harder to reproduce).
[how]
Do query in goroutine, wait when interruption done.
[testing]
Add unit test that reproduces error cases.
