[switch] start and stop accepting connections

[pgte]

For each different transport individually, in order to do connection throttling, it's important to be able to start and stop the listener by command of the user.

[pgte]

Also depends on fixing these issues:

• closing the listener drops underlying connections js-libp2p-websocket-star#49
• closing the listener drops underlying connections js-libp2p-tcp#92

[pgte]

Also the UTP transport has the same issue:

• start and stop accepting connections js-libp2p-utp#76

[pgte]

@diasdavid there's an incongruence in the interface-transport docs:

options is an optional object that might contain the following properties:

• timeout - A timeout value (in ms) that fires and destroys all the connections on this transport if the transport is not able to close graciously. (e.g { timeout: 1000 })

[jacobheun]

We will also need to remove/change the interface-stream-muxer test at https://github.com/libp2p/interface-stream-muxer/blob/v0.5.9/src/close-test.js#L37, as it expects the muxer connections to close when the listener is closed. Instead it should validate the connection is still usable.