make sure to unset batch_key when connection is removed from batch

[jkarneges]

For bulk IPC communication, Pushpin will address multiple connections in the same packet. For example, if a component needs to send keep-alive packets for 100 connections with a peer, then it will try to send just 1 keep-alive packet to that peer that is addressed to all 100 connections.

In connmgr, the Batch type helps sort out how to do this. When it is time to perform such bulk communications, connections are added to the batch with Batch::add, even if the connections are with different peers. Then, peer-specific "groups" of connections can be extracted with Batch::take_group for easily sending to them.

When a ConnectionItem is added to a batch, its batch_key field will be set to the return value of Batch::add. This value is kept around so if the connection is dropped it can remove itself from the batch with Batch::remove. Importantly, when the connection has been removed from the batch via Batch::take_group, this field needs to be set to None. In some very rare cases we might fail to do this, which means when the connection is eventually dropped it will try to remove itself from a batch that it's not in. This will result in a panic in List::remove().

Those rare cases we fail to unset batch_key are:

• If the closure provided to take_group returns None. In this case, the connection will still be removed from the batch without this being communicated to the caller, and so the caller doesn't know to unset the batch_key.
• If the caller errors out in some way between the time take_group returns and when it unsets batch_key.

This PR makes the batch API a little more clear and ensures batch_key is always set to None for connections removed by take_group.