Maximize the amount of concurrency while chunking.

In order to reduce our amount of backpressure we need to limit the
amount of concurrent chunk requests we can have so the gateway buffer
has some time to breathe.

discord/state.py

@@ -354,7 +354,6 @@ class ConnectionState:
                     except asyncio.TimeoutError:
                         break
                     else:
-
                         if self._guild_needs_chunking(guild):
                             future = await self.chunk_guild(guild, wait=False)
                             states.append((guild, future))
@@ -1044,6 +1043,8 @@ class AutoShardedConnectionState(ConnectionState):
     async def _delay_ready(self):
         await self.shards_launched.wait()
         processed = []
+        max_concurrency = len(self.shard_ids) * 2
+        current_bucket = []
         while True:
             # this snippet of code is basically waiting N seconds
             # until the last GUILD_CREATE was sent
@@ -1053,8 +1054,19 @@ class AutoShardedConnectionState(ConnectionState):
                 break
             else:
                 if self._guild_needs_chunking(guild):
+                    log.debug('Guild ID %d requires chunking, will be done in the background.', guild.id)
+                    if len(current_bucket) >= max_concurrency:
+                        try:
+                            await utils.sane_wait_for(current_bucket, timeout=max_concurrency * 10)
+                        except asyncio.TimeoutError:
+                            fmt = 'Shard ID %s failed to wait for chunks from a sub-bucket with length %d'
+                            log.warning(fmt, self.shard_id, len(current_bucket))
+                        finally:
+                            current_bucket = []
+
                     # Chunk the guild in the background while we wait for GUILD_CREATE streaming
                     future = asyncio.ensure_future(self.chunk_guild(guild))
+                    current_bucket.append(future)
                 else:
                     future = self.loop.create_future()
                     future.set_result(True)