refactor connection counter (#343)

* Remove restart_worker test

* Remove Slab

* Rework counter

* Make counter limit switch accurate

* Remove backpressure. Add pause state

* make changes for review

* fix doc comment for counter

Author: fakeshadow

actix-server/Cargo.toml

@@ -28,7 +28,6 @@ futures-core = { version = "0.3.7", default-features = false, features = ["alloc
 log = "0.4"
 mio = { version = "0.7.6", features = ["os-poll", "net"] }
 num_cpus = "1.13"
-slab = "0.4"
 tokio = { version = "1.2", features = ["sync"] }
 
 [dev-dependencies]