Add lock to prevent parallel reloads
Add lock to prevent parallel reloads

Parallel reloads might cause haproxy to leave old processes around.
This closes #5.

knesenko/marathon-lb,matt-deboer/marathon-lb,knesenko/marathon-lb,GrovoLearning/marathon-lb,mesosphere/marathon-lb,mesosphere/marathon-lb,vixns/marathon-lb,JamesAwesome/marathon-lb,matt-deboer/marathon-lb,mesosphere/marathon-lb,GrovoLearning/marathon-lb,vixns/marathon-lb,GrovoLearning/marathon-lb,vixns/marathon-lb,JamesAwesome/marathon-lb,matt-deboer/marathon-lb,knesenko/marathon-lb,JamesAwesome/marathon-lb