[commands] Make CooldownMapping.get_bucket take Message instead.

Requiring a full blown Context might be a bit overkill considering
we only use a single attribute from it.

discord/ext/commands/cooldowns.py

@@ -103,8 +103,7 @@ class CooldownMapping:
     def from_cooldown(cls, rate, per, type):
         return cls(Cooldown(rate, per, type))
 
-    def _bucket_key(self, ctx):
-        msg = ctx.message
+    def _bucket_key(self, msg):
         bucket_type = self._cooldown.type
         if bucket_type is BucketType.user:
             return msg.author.id
@@ -122,12 +121,12 @@ class CooldownMapping:
         for k in dead_keys:
             del self._cache[k]
 
-    def get_bucket(self, ctx):
+    def get_bucket(self, message):
         if self._cooldown.type is BucketType.default:
             return self._cooldown
 
         self._verify_cache_integrity()
-        key = self._bucket_key(ctx)
+        key = self._bucket_key(message)
         if key not in self._cache:
             bucket = self._cooldown.copy()
             self._cache[key] = bucket

discord/ext/commands/core.py

@@ -438,7 +438,7 @@ class Command:
         yield from self._verify_checks(ctx)
 
         if self._buckets.valid:
-            bucket = self._buckets.get_bucket(ctx)
+            bucket = self._buckets.get_bucket(ctx.message)
             retry_after = bucket.update_rate_limit()
             if retry_after:
                 raise CommandOnCooldown(bucket, retry_after)
@@ -462,7 +462,7 @@ class Command:
         if not self._buckets.valid:
             return False
 
-        bucket = self._buckets.get_bucket(ctx)
+        bucket = self._buckets.get_bucket(ctx.message)
         return bucket.get_tokens() == 0
 
     def reset_cooldown(self, ctx):
@@ -474,7 +474,7 @@ class Command:
             The invocation context to reset the cooldown under.
         """
         if self._buckets.valid:
-            bucket = self._buckets.get_bucket(ctx)
+            bucket = self._buckets.get_bucket(ctx.message)
             bucket.reset()
 
     @asyncio.coroutine