Roman Leventov

@gaborbernat what do you think about using [SQLite's VFS interface](https://www.sqlite.org/vfs.html) directly, without the "dummy database" thing? https://github.com/rogerbinns/apsw provides bindings

@gaborbernat I've added tests. If you want to finesse API (context managers vs. parameters to acquire_read() and acquire_write() methods; are read_lock() and write_lock() methods even needed? Are instance's default timeout...

@gaborbernat I don't need this PR to be merged to use it later through `filelock` library. In fact, I submitted a PR only to attract code review and discussion. Myself,...