Add support for sqlite3_interrupt

[briangreenery]

This change adds support for sqlite3_interrupt. This makes it possible to interrupt a long running query.

I called the function synchronously instead of going through db->Schedule because if it was scheduled it wouldn't be possible to interrupt exec. This is because while exec is running the work queue is not processed. Also, the sqlite3_interrupt function doesn't do any I/O or acquire any locks, so it won't block the javascript thread.

The downside of calling sqlite3_interrupt synchronously means that it has to check whether the database is open, and has to check that sqlite3_close is not about to be run by another thread. Basically, if sqlite3_interrupt is called on an already closed database, then it could cause memory corruption.

To avoid this, I added a closing variable that's set right before sending a close to a worker thread. The open variable isn't sufficient because it's only set to false after sqlite3_close has been called.

[hrundik]

@springmeyer : my team is also interested in this feature. Do you plan to merge this PR? What is the obstacle of not merging?

[panther7]

Hi, please add this to docs API. Thanks