chore: switch database engine to sqlite (fixes #9954) (#9965)

Switch the database from LevelDB to SQLite, for greater stability and
simpler code.

Co-authored-by: Tommy van der Vorst <tommy@pixelspark.nl>
Co-authored-by: bt90 <btom1990@googlemail.com>

gui/default/syncthing/core/aboutModalView.html

@@ -59,9 +59,12 @@ Jakob Borg, Audrius Butkevicius, Jesse Lucas, Simon Frei, Tomasz Wilczyński, Al
           <li><a href="https://github.com/golang/protobuf">golang/protobuf</a>, Copyright &copy; 2010 The Go Authors.</li>
           <li><a href="https://github.com/golang/snappy">golang/snappy</a>, Copyright &copy; 2011 The Snappy-Go Authors.</li>
           <li><a href="https://github.com/jackpal/gateway">jackpal/gateway</a>, Copyright &copy; 2010 Jack Palevich.</li>
+          <li><a href="https://github.com/jmoiron/sqlx">jmoiron/sqlx</a>, Copyright &copy; 2013 Jason Moiron.</li>
           <li><a href="https://github.com/kballard/go-shellquote">kballard/go-shellquote</a>, Copyright &copy; 2014 Kevin Ballard.</li>
           <li><a href="https://github.com/mattn/go-isatty">mattn/go-isatty</a>, Copyright &copy; Yasuhiro MATSUMOTO.</li>
+          <li><a href="https://github.com/mattn/go-sqlite3">mattn/go-sqlite3</a>, Copyright &copy; 2014 Yasuhiro Matsumoto</li>
           <li><a href="https://github.com/matttproud/golang_protobuf_extensions">matttproud/golang_protobuf_extensions</a>, Copyright &copy; 2012 Matt T. Proud.</li>
+          <li><a href="https://modernc.org/sqlite">modernc.org/sqlite</a>, Copyright &copy; 2017 The Sqlite Authors</li>
           <li><a href="https://github.com/oschwald/geoip2-golang">oschwald/geoip2-golang</a>, Copyright &copy; 2015, Gregory J. Oschwald.</li>
           <li><a href="https://github.com/oschwald/maxminddb-golang">oschwald/maxminddb-golang</a>, Copyright &copy; 2015, Gregory J. Oschwald.</li>
           <li><a href="https://github.com/petermattis/goid">petermattis/goid</a>, Copyright &copy; 2015-2016 Peter Mattis.</li>