remove databases, use SQLAlechemy 2.0 instead

Among the many changes we switch to using SQLAlchemy's connection pool,
which means we are no longer required to guard against multiple threads
working on the database.
All db funcs now receive a connection to use as their first argument,
this allows the caller to control transaction & rollback behavior.

scripts/tests

@@ -6,7 +6,7 @@ dbfile="${UNWIND_DATA:-./data}/tests.sqlite"
 
 # Rollback in Databases is currently broken, so we have to rebuild the database
 # each time; see https://github.com/encode/databases/issues/403
-trap 'rm "$dbfile"' EXIT TERM INT QUIT
+trap 'rm "$dbfile" "${dbfile}-shm" "${dbfile}-wal"' EXIT TERM INT QUIT
 
 [ -z "${DEBUG:-}" ] || set -x