net: add MAX_PEERS, add idle timeouts

[dr-frmr]

Problem

networking module does not track number of pipes used, leading to potential overuse in combination with file-handler modules.

Solution

add infra to net:distro:sys that tracks number of open connections, enables closing excess ones

• idle timeouts for open connections
• cap on max active peers, close longest-idle peer connection when hit
• etc etc

Testing

highly involved - test networking at load under number of configs

Docs Update

TODO