feat: pre-update hooks

See merge request !80

This is useful in the following situation:

* `query` function gets called
* SQL query is executed correctly, the `r` result (Rows) is set
* after that, but before `query` returns, `ctx` is canceled
* `interruptOnDone`’s goroutine catches the cancelation and signals
  the completion back to `query` by turning the atomic signaling
  variable `done` to 1
* `query` finishes and returns
* the anonymous deferred function patched here is popped off the
  deferred stack and executed

The current code, seeing that `done` is not zero, will overwrite `r`
(Rows) with `nil`, and that implies the Rows can never be `Close`d.

Unclosed Rows mean that read transactions never finish, and that
blocks the checkpointer process and cause the WAL to grow unbounded,
and other read queries to slow down unbounded.

https://sqlite.org/wal.html#avoiding_excessively_large_wal_files
("Checkpoint Starvation")

Signed-off-by: Silvio Moioli <silvio@moioli.net>

Signed-off-by: Silvio Moioli <silvio@moioli.net>
Co-authored-by: Alejandro Ruiz <alejandro.ruiz@suse.com>

Close Rows object if the context is canceled before deferred function

See merge request !81

- Introduce vtab subpackage (modernc.org/sqlite/vtab) exposing Module, Table,...

See merge request !84

The previous code was subject to a TOCTOU race:
the atomic.AddInt32 in the select statement could succeed,
but then context switch away before actually calling c.interrupt.
The actual call to c.interrupt could end up occurring later,
interrupting an unrelated subsequent query.

Fixes #241

fix TOCTOU interrupt race

Closes #241

See merge request !86