Opening a substream may block the endpoint for significant time

[da-kami]

This is an overall problem, but becomes severe on the maker when the maker opens substreams with takers that are not availble (for whatever reason). The endpoint handler for OpenSubstream then only times out after 20s, blocking the complete endpoint while waiting.

Since the maker loops over all "connected" takers when opening substreams for ping and offers, and it can run into this problem with some takers, the blockage then might be so long that other takers lose connections because ping might not be handled.