Fix Potential crash on db close

[stevensJourney]

Overview

This adds extra checks and mutex locks to the .close method. This should prevent some cases where closing a database might crash the application.

This PR includes 2 fixes which should help with crashes.

The first fix involves clearing callbacks for the SQLite commit and table change hooks on close. These hooks are handled asynchronously and are called via the React native invoker. A race condition could occur where an async callback is executed after the connection has closed and memory has been freed. The callback handlers now use safe shared pointer which is used to check if the connection is closed - if closed the callback execution is ended.

The second fix involves improving the waitForFinished logic has which occurs on close. This should prevent SQLite operations on a closed database. The JS write and read locks will now also reject any pending requests if the database has been closed.

Units tests have been added to ensure the close method works correctly.

Tests pass on Android and iOS locally.

TODOs

• The iOS test app seems to be broken.

[rkistner]

Looks good overall, but did not check all the concurrency logic in detail.