server: fix FSMHandler's holdTimer race
server: fix FSMHandler's holdTimer race

FSMHandler's holdTimer could be accessed by rx goroutine before it's
initialized. Let's use channel rather than sharing time.Timer pointer.

Signed-off-by: FUJITA Tomonori <93dac1fe9c4b2a3957982200319981492ad4976e@lab.ntt.co.jp>

a16/gobgp,fujita/gobgp,a16/gobgp,fujita/gobgp,osrg/gobgp,fujita/gobgp,fujita/gobgp,osrg/gobgp,Dipsingh/gobgp,h-naoto/gobgp,osrg/gobgp,tamihiro/gobgp,a16/gobgp,a16/gobgp,YujiOshima/gobgp,ishidawataru/gobgp,a16/gobgp,postfix/gobgp,a16/gobgp,ramrunner/gobgp,tamihiro/gobgp,YujiOshima/gobgp,postfix/gobgp,a16/gobgp,tamihiro/gobgp,h-naoto/gobgp,h-naoto/gobgp,ramrunner/gobgp,Dipsingh/gobgp,j-kato/gobgp,Dipsingh/gobgp,tamihiro/gobgp,ramrunner/gobgp,j-kato/gobgp,j-kato/gobgp,YujiOshima/gobgp,postfix/gobgp,osrg/gobgp,ishidawataru/gobgp,ishidawataru/gobgp