sync: add mpsc::Receiver::blocking_recv_many

[barafael]

Fixes: #6865

Motivation

I would like to use an API like recv_many, but from a blocking context.

Solution

Use crate::future::block_on in a similar fashion to how blocking_recv does it to call the async recv_many function.

What's missing

Tests and docs. I'll complete them once it's clear if this entire change makes sense. Do you think referring to blocking_recv and recv_many from the docs is fine or should there be more specific docs?

[Darksonn]

Thanks.