Do not cancel context for async queries on success

[carlosms]

Fix #857.

The code in #806 added a context cancel call when the query handle finishes,

go-mysql-server/server/handler.go

Line 117 in 68b087c

defer cancel()

For an async index creation the driver is called in a goroutine

go-mysql-server/sql/plan/create_index.go

Line 173 in 68b087c

createIndex := func() {

And when the driver went to do the actual index creation in that goroutine, the query had already exited, and the context was cancelled

go-mysql-server/sql/index/pilosa/driver.go

Line 419 in 68b087c

case <-ctx.Done():

The first option discussed in slack (private link) was to change the context of the createIndex function linked above with a ctx.WithContext(context.Background()), if the call was made in a goroutine.

This removes the original context cancellation, which fixes the problem, but it also prevents from doing a KILL QUERY of the async task afterwards. This would be a regression over the latest stable gitbase release.

The code I propose in this PR instead removes the deferred context cancellation in the handler if the query is async. It feels like the code is kind of mixing responsibilities, making the Handler aware of details about the query that should be handled by the Engine. But this was the cleanest approach I could think of.

I am still trying to figure out how this could be tested, any suggestion would be welcome.