fix: close stream after sending identify

[MarcoPolo]

Close the stream after we finish writing the identify message. This is behavior is specified https://github.com/libp2p/specs/blob/master/identify/README.md#identify

The peer being identified responds by returning an Identify message and closes the stream.

This shows up when interacting with a go-libp2p node as go-libp2p waits for the stream to close before finishing reading the messages: https://github.com/libp2p/go-libp2p/blob/master/p2p/protocol/identify/id.go#L455.

[MarcoPolo]

bit more context, this is particularly bad when interacting with go-libp2p nodes as go-libp2p will wait for identify to finish before continuing other protocols. So we are blocked until the identify timeout happens before we can do anything else when interacting a js-libp2p node with a go-libp2p node.

For why go does this multi-message merge, see: libp2p/go-libp2p#954 (comment)

[achingbrain]

Good catch, thanks!