substreams: Allow substreams to close without consuming self

[lexnv]

The current implementation consumes self when calling the close method on the substream.
After the close is called, the substream's end user should no longer write data into it.
This is a bit restrictive because the substream can be drained/read after the close call.

litep2p/src/substream/mod.rs

Line 339 in 2a49fd0

pub async fn close(self) {

It is entirely possible to have the following scenario:

• Write data to the substream
• Substream close (signals the receiver end via read bytes number = 0)
• Read data from the substream

One such implementation is the /perf/1.0.0 protocol from rust-libp2p.