Switch sqlite3 journal mode to WAL.

WAL mode does make the deadlocking on MacOS go away.  This suggests
that pysqlite3 was leaving *read* transactions open for a long time; in
old-style sqlite journals, this prevents anyone from obtaining a write
lock, although it doesn't prevent other concurrent reads.

With WAL journals, writes can happen even while readers are holding a
lock, but the journal doesn't flush until the readers have released it.
This is not a "real" fix but it's fairly harmless, since all redo
instances will exit eventually, and when they do, the WAL journal can
be flushed.

state.py

@@ -14,7 +14,7 @@ STAMP_MISSING='0'   # the stamp of a nonexistent file
 def _connect(dbfile):
     _db = sqlite3.connect(dbfile, timeout=TIMEOUT)
     _db.execute("pragma synchronous = off")
-    _db.execute("pragma journal_mode = PERSIST")
+    _db.execute("pragma journal_mode = WAL")
     _db.text_factory = str
     return _db