app-layer: add flow counters

app-layer: add flow counters

This adds per flow counters for all
supported protocols.

This results in new data in stats output that looks like:
```
    "app-layer": {
      "flow": {
        "http": 9310,
        "ftp": 0,
        "smtp": 0,
        "tls": 71,
        "ssh": 0,
        "imap": 0,
        "msn": 0,
        "smb": 170,
        "dcerpc_udp": 0,
        "dns_udp": 870,
        "dcerpc_tcp": 2,
        "dns_tcp": 0
      },
    },
```