pimd: Add `clear ip mroute [vrf NAME] count` command to pim

pimd: Add `clear ip mroute [vrf NAME] count` command to pim

When debugging a large number of mroutes and data is changing
fast it is sometimes hard to know what has actually changed.
Add a `clear ip mroute count` command that resets the last
data points gathered and allows you to see what has changed
since the last clear.

Output:
donna.cumulusnetworks.com# show ip mroute count

Source          Group           LastUsed Packets Bytes WrongIf
*               224.0.1.60      272      0       0          0
10.50.11.11     239.255.255.250 2        1       203        1
10.50.11.13     239.255.255.250 66       1       203        1
10.50.11.100    239.255.255.250 68       1       203        1
10.50.11.114    239.255.255.250 62       2       406        1
10.50.11.129    239.255.255.250 10       1       199        1
10.50.11.143    239.255.255.250 44       1       203        1
10.50.11.144    239.255.255.250 44       1       203        1
10.50.11.156    239.255.255.250 66       1       203        1
10.50.11.235    239.255.255.250 149      0       0          0
10.50.11.246    239.255.255.250 54       5       965        1
donna.cumulusnetworks.com# clear ip mroute count
donna.cumulusnetworks.com# show ip mroute count

Source          Group           LastUsed Packets Bytes WrongIf
*               224.0.1.60      279      0       0          0
10.50.11.11     239.255.255.250 9        0       0          0
10.50.11.13     239.255.255.250 73       0       0          0
10.50.11.100    239.255.255.250 76       0       0          0
10.50.11.114    239.255.255.250 69       0       0          0
10.50.11.129    239.255.255.250 17       0       0          0
10.50.11.143    239.255.255.250 51       0       0          0
10.50.11.144    239.255.255.250 51       0       0          0
10.50.11.156    239.255.255.250 73       0       0          0
10.50.11.235    239.255.255.250 156      0       0          0
10.50.11.246    239.255.255.250 61       0       0          0
donna.cumulusnetworks.com# show ip mroute count

Source          Group           LastUsed Packets Bytes WrongIf
*               224.0.1.60      300      0       0          0
10.50.11.11     239.255.255.250 30       0       0          0
10.50.11.13     239.255.255.250 94       0       0          0
10.50.11.100    239.255.255.250 96       0       0          0
10.50.11.114    239.255.255.250 90       0       0          0
10.50.11.119    239.255.255.250 7        1       203        1
10.50.11.127    239.255.255.250 3        2       406        1
10.50.11.129    239.255.255.250 38       0       0          0
10.50.11.143    239.255.255.250 72       0       0          0
10.50.11.144    239.255.255.250 72       0       0          0
10.50.11.156    239.255.255.250 94       0       0          0
10.50.11.197    239.255.255.250 2        1       200        1
10.50.11.235    239.255.255.250 177      0       0          0
10.50.11.246    239.255.255.250 82       0       0          0

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>