Concurrent upgrades (#489)

* adding an upgraded event to conn

* set stopped flag asap

* trigger upgradded event on conn

* set concurrency limit for accepts

* backporting semaphore from tcp-limits2

* export unittests module

* make params explicit

* tone down debug logs

* adding semaphore tests

* use semaphore to throttle concurent upgrades

* add libp2p scope

* trigger upgraded event before any other events

* add event handler for connection upgrade

* cleanup upgraded event on conn close

* make upgrades slot release rebust

* dont forget to release slot on nil connection

* misc

* make sure semaphore is always released

* minor improvements and a nil check

* removing unneeded comment

* make upgradeMonitor a non-closure proc

* make sure the `upgraded` event is initialized

* handle exceptions in accepts when stopping

* don't leak exceptions when stopping accept loops

libp2p/protocols/secure/secure.nim

@@ -46,6 +46,7 @@ proc init*(T: type SecureConn,
              peerInfo: peerInfo,
              observedAddr: observedAddr,
              closeEvent: conn.closeEvent,
+             upgraded: conn.upgraded,
              timeout: timeout,
              dir: conn.dir)
   result.initStream()