pimd: Implement watermark warning for igmp group count and add igmp group count

pimd: Implement watermark warning for igmp group count and add igmp group count

This CLI will allow user to configure a igmp group limit which will generate
a watermark warning when reached.
Though watermark may not make sense without setting a limit, this
implementation shall serve as a base to implementing limit in future and helps
tracking a particular scale currently.

Testing:
=======

ip igmp watermark-warn <10-60000>

on reaching the configured number of group, pim will issue warning

2019/09/18 18:30:55 PIM: SCALE ALERT: igmp group count reached watermak limit: 210(vrf: default)

Also added group count and watermark limit configured on cli - show ip igmp groups [json]

<snip>

Sw3# sh ip igmp groups json
{
  "Total Groups":221,  <=====
  "Watermark limit":210, <=========
  "ens224":{
    "name":"ens224",
    "state":"up",
    "address":"40.0.0.1",
    "index":6,
    "flagMulticast":true,
    "flagBroadcast":true,
    "lanDelayEnabled":true,
    "groups":[
      {
        "source":"40.0.0.1",
        "group":"225.1.1.122",
        "timer":"00:03:56",
        "sourcesCount":1,
        "version":2,
        "uptime":"00:00:24"

<\snip>

<snip>

Sw3(config)# do sh ip igmp group
Total IGMP groups: 221
Watermark warn limit(Set) : 210
Interface        Address         Group           Mode Timer    Srcs V Uptime
ens224           40.0.0.1        225.1.1.122     ---- 00:04:06    1 2 00:13:22
ens224           40.0.0.1        225.1.1.144     ---- 00:04:02    1 2 00:13:22
ens224           40.0.0.1        225.1.1.57      ---- 00:04:01    1 2 00:13:22
ens224           40.0.0.1        225.1.1.210     ---- 00:04:06    1 2 00:13:22

<\snip>

Signed-off-by: Saravanan K <saravanank@vmware.com>