## What?

Prior to this commit connecting 40k AMQP clients with 5 sessions each,
i.e. 200k sessions in total, took 7m55s.

After to this commit the same scenario takes 1m37s.

Additionally, prior to this commit, disconnecting all connections and sessions
at once caused the pg process to become overloaded taking ~14 minutes to
process its mailbox.

After this commit, these same deregistrations take less than 5 seconds.

To repro:
```go

package main

import (
	"context"
	"log"
	"time"

	"github.com/Azure/go-amqp"
)

func main() {
	for i := 0; i < 40_000; i++ {
		if i%1000 == 0 {
			log.Printf("opened %d connections", i)
		}
		conn, err := amqp.Dial(
			context.TODO(),
			"amqp://localhost",
			&amqp.ConnOptions{SASLType: amqp.SASLTypeAnonymous()})
		if err != nil {
			log.Fatal("open connection:", err)
		}
		for j := 0; j < 5; j++ {
			_, err = conn.NewSession(context.TODO(), nil)
			if err != nil {
				log.Fatal("begin session:", err)
			}
		}
	}
	log.Println("opened all connections")
	time.Sleep(5 * time.Hour)
}
```

 ## How?

This commit uses separate pg scopes (that is processes and ETS tables) to register
AMQP connections and AMQP sessions. Since each Pid is now its own group,
registration and deregistration is fast.