Allow Topics to close any handles they have

[aschmahmann]

@Stebalien mentioned in libp2p/go-libp2p-pubsub-router#37 (comment) that the restrictions posed on closing a Topic handle shouldn't need to exist

go-libp2p-pubsub/topic.go

Line 187 in 01b9825

// Close closes down the topic. Will return an error unless there are no active event handlers or subscriptions.

IIUC the reasoning is that since only one valid Topic handle can exist for a given topic that when that handler calls Close() it is willing to kill any existing Subscription or TopicEventHandler objects that are outstanding.

This may clean up some code, by removing a bunch of error checking/handling, but comes at the cost of adding complexity for a Topic with N Subscriptions that only wants to close when all N Subscriptions are closed.

Both options are by me. WDYT @vyzo?

[vyzo]

Maybe we can simply reference count the closures?

[Stebalien]

As we decided in #198, topic handles are now owned by a single service. There shouldn't be any need to reference count unless I'm missing something.