internal: drop graceful shutdown feature flag

Graceful shutdown was introduced in Tarantool with [1]. A week later it
was reworked with [2]. So the timeline is as follows.

- Graceful shutdown is introduced, new feature flag is added and
  the protocol version is bumped to 4.
- Graceful shutdown is reworked, feature flag is dropped and the
  protocol version is dropped to 3.
- Next Tarantool version released.

So the idea for users is "IPROTO_FEATURE_GRACEFUL_SHUTDOWN and
protocol version 4 never existed". It also means that any new feature
flag may have value `4` (same as IPROTO_FEATURE_GRACEFUL_SHUTDOWN), but
with completely different meaning. IPROTO_FEATURE_GRACEFUL_SHUTDOWN flag
was added in [3] by mistake due to misunderstanding the event timeline.
This patch removes the flag. Since `_features` dict is an internal of a
connection object, the change shouldn't break anything for users.

1. tarantool/tarantool@6f29f9d
2. tarantool/tarantool@2e9cbec
3. #243

Closes #262

tarantool/connection.py

@@ -63,7 +63,6 @@
     IPROTO_FEATURE_TRANSACTIONS,
     IPROTO_FEATURE_ERROR_EXTENSION,
     IPROTO_FEATURE_WATCHERS,
-    IPROTO_FEATURE_GRACEFUL_SHUTDOWN,
     IPROTO_CHUNK,
 )
 from tarantool.error import (
@@ -514,7 +513,6 @@ def __init__(self, host, port,
             IPROTO_FEATURE_TRANSACTIONS: False,
             IPROTO_FEATURE_ERROR_EXTENSION: False,
             IPROTO_FEATURE_WATCHERS: False,
-            IPROTO_FEATURE_GRACEFUL_SHUTDOWN: False,
         }
 
         if connect_now:

tarantool/const.py

@@ -96,7 +96,6 @@
 IPROTO_FEATURE_TRANSACTIONS = 1
 IPROTO_FEATURE_ERROR_EXTENSION = 2
 IPROTO_FEATURE_WATCHERS = 3
-IPROTO_FEATURE_GRACEFUL_SHUTDOWN = 4
 
 # Default value for connection timeout (seconds)
 CONNECTION_TIMEOUT = None

test/suites/test_protocol.py

@@ -13,7 +13,6 @@
     IPROTO_FEATURE_TRANSACTIONS,
     IPROTO_FEATURE_ERROR_EXTENSION,
     IPROTO_FEATURE_WATCHERS,
-    IPROTO_FEATURE_GRACEFUL_SHUTDOWN,
 )
 
 class TestSuite_Protocol(unittest.TestCase):
@@ -86,7 +85,6 @@ def test_04_protocol(self):
         self.assertEqual(self.con._features[IPROTO_FEATURE_STREAMS], False)
         self.assertEqual(self.con._features[IPROTO_FEATURE_TRANSACTIONS], False)
         self.assertEqual(self.con._features[IPROTO_FEATURE_WATCHERS], False)
-        self.assertEqual(self.con._features[IPROTO_FEATURE_GRACEFUL_SHUTDOWN], False)
 
     @classmethod
     def tearDownClass(self):