Allow for alternative Connection Managers

[maschad]

Currently Libp2p enforces the DefaultConnectionManager for managing all connections. Whilst it's quite configurable, there are particular use cases in which it is a bit inflexible and a consumer may want to have complete control over the process of opening and closing connections as well as pruning and dialling peers. This could be enabled by passing a module which implements the connection-manager-interface to the Libp2p instance, otherwise it would default to the DefaultConnectionManager with the default config.

const libp2p = await createLibp2p({
  transports: [
    tcp()
  ],
 connectionManager: [
	myConnectionManager({
 		maxConnections: Infinity,
    	minConnections: 0,
    	pollInterval: 2000
          // other configurations
	})
 ]
})

[maschad]

Closed in favour of #1563 which will allow a consumer to define arbitrary services that have access to libp2p internal components