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