Hard limit on file descriptors

[Stebalien]

The swarm should set a hard limit on new file descriptor consuming connections at 90% of the file descriptor limit. Otherwise, things break terribly.

[Jorropo]

I've planned to do it in Quality Transport. The struct returned by QTransport.Score(...) Score have an Fd field, transport can return the number of Fd used to open this connection, if not enough Fd are avaible the connection is gonna be put on hold.

[Stebalien]

We already track which transports consume file descriptors, that's not the issue. This issue is simply about setting a hard upper limit.

[marten-seemann]

Closing in favor of libp2p/go-libp2p#1260.