Replace filter execution control with bools instead of exceptions

tests/test_filter.py

@@ -31,7 +31,6 @@ class TestFilter(unittest.TestCase):
         self.path = tempfile.mkdtemp()
         self.store = SyncFsStore(self.path)
         self.session = SyncSession(self.store)
-        self.session.start()
         self.conn = MockConn()
 
 
@@ -45,6 +44,8 @@ class TestFilter(unittest.TestCase):
         fltr_two = MockFilter('bar')
         self.store.register(fltr_two)
 
+        self.session.start()
+
         tx_hash = os.urandom(32).hex()
         tx = MockTx(42, tx_hash)
         block = MockBlock(13, [tx_hash])
@@ -61,6 +62,8 @@ class TestFilter(unittest.TestCase):
         fltr_two = MockFilter('bar')
         self.store.register(fltr_two)
 
+        self.session.start()
+
         tx_hash = os.urandom(32).hex()
         tx = MockTx(42, tx_hash)
         block = MockBlock(13, [tx_hash])