ssntp: Take a lock before modifying server.listener
ssntp: Take a lock before modifying server.listener

There's nothing preventing us from calling Stop()
and Serve() at the same time, leading to a potential
race on server.listener.

Fixes #183

Signed-off-by: Samuel Ortiz <0ba86cb3f08bbb861958e54bd3438887adb4263c@linux.intel.com>

tpepper/ciao,mrkz/ciao,01org/ciao,rbradford/ciao,obedmr/ciao,mrkz/ciao,ciao-project/ciao,leoswaldo/ciao,matthewrsj/ciao,ciao-project/ciao,kaccardi/ciao,leoswaldo/ciao,erick0z/ciao,erick0z/ciao,obedmr/ciao,markdryan/ciao,matthewrsj/ciao,wcwxyz/ciao,mcastelino/ciao,mcastelino/ciao,matthewrsj/ciao,wcwxyz/ciao,mrkz/ciao,rbradford/ciao,wcwxyz/ciao,erick0z/ciao,obedmr/ciao,markdryan/ciao,jorgevgut/ciao,mcastelino/ciao,leoswaldo/ciao,jorgevgut/ciao,jorgevgut/ciao,01org/ciao,tpepper/ciao,kaccardi/ciao,tpepper/ciao