Fix race condition by using lock on enable_radvd
Fix race condition by using lock on enable_radvd

For an HA Router, radvd is spawned only in the Master Node.
KeepalivedStateChangeHandler after receiving the state notification
on the unix domain socket, would either enable/disable the radvd
process depending on the state of the HA Router. In parallel, if
there is a request to add an interface to the router, router.process()
would try to enable radvd in parallel. So there is a possible race
condition as two threads try to enable radvd in parallel. This patch
uses appropriate locking mechanism to prevent this race condition.

Closes-Bug: #1466663
Change-Id: I40df78cc57fcde5512336d9dfef88b3b06a69bcb

neoareslinux/neutron,cloudbase/neutron,huntxu/neutron,jacknjzhou/neutron,bigswitch/neutron,sasukeh/neutron,shahbazn/neutron,barnsnake351/neutron,openstack/neutron,igor-toga/local-snat,yanheven/neutron,suneeth51/neutron,noironetworks/neutron,takeshineshiro/neutron,bigswitch/neutron,paninetworks/neutron,mahak/neutron,mattt416/neutron,javaos74/neutron,mmnelemane/neutron,suneeth51/neutron,shahbazn/neutron,glove747/liberty-neutron,vivekanand1101/neutron,klmitch/neutron,sebrandon1/neutron,openstack/neutron,glove747/liberty-neutron,skyddv/neutron,takeshineshiro/neutron,sasukeh/neutron,eayunstack/neutron,huntxu/neutron,neoareslinux/neutron,wolverineav/neutron,cisco-openstack/neutron,openstack/neutron,mahak/neutron,igor-toga/local-snat,mahak/neutron,SamYaple/neutron,jumpojoy/neutron,cisco-openstack/neutron,dims/neutron,MaximNevrov/neutron,apporc/neutron,klmitch/neutron,chitr/neutron,apporc/neutron,swdream/neutron,skyddv/neutron,SamYaple/neutron,adelina-t/neutron,swdream/neutron,watonyweng/neutron,barnsnake351/neutron,silenci/neutron,paninetworks/neutron,vivekanand1101/neutron,chitr/neutron,yanheven/neutron,watonyweng/neutron,dhanunjaya/neutron,asgard-lab/neutron,cloudbase/neutron,noironetworks/neutron,JianyuWang/neutron,MaximNevrov/neutron,sebrandon1/neutron,jumpojoy/neutron,adelina-t/neutron,jacknjzhou/neutron,dims/neutron,asgard-lab/neutron,mmnelemane/neutron,silenci/neutron,dhanunjaya/neutron,wolverineav/neutron,JianyuWang/neutron,javaos74/neutron,mattt416/neutron,eayunstack/neutron