Allow Topics to close any handles they have

Nobody has claimed this yet.

@Stebalien mentioned in https://github.com/libp2p/go-libp2p-pubsub-router/pull/37#discussion_r345971256 that the restrictions posed on closing a Topic handle shouldn't need to exist https://github.com/libp2p/go-libp2p-pubsub/blob/01b9825fbee1848751d90a8469e3f5f43bac8466/topic.go#L187

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 `Subscription`s that only wants to close when all N `Subscription`s are closed.

Both options are by me. WDYT @vyzo?

Start with topic.go around line 187 and read the linked discussion in pull request 37 to understand the current restrictions on closing Topic handles. Compare the proposed behavior with the existing Subscription and TopicEventHandler lifecycle, then confirm the desired API semantics with maintainers before identifying tests or implementation changes.

Written by the indexing model from the issue text.