api/grpc_server: Avoid name collision "server"
api/grpc_server: Avoid name collision "server"

To improve code inspection result, this patch renames "server"
variables in NewServer() to "s" because "server" collides the
imported package name "github.com/osrg/gobgp/server".

Signed-off-by: IWASE Yusuke <bb80e84cc9f5a8f01385872ee89032beb5d4eba7@gmail.com>

osrg/gobgp,fujita/gobgp,a16/gobgp,osrg/gobgp,a16/gobgp,tamihiro/gobgp,a16/gobgp,j-kato/gobgp,j-kato/gobgp,a16/gobgp,tamihiro/gobgp,a16/gobgp,fujita/gobgp,tamihiro/gobgp,j-kato/gobgp,a16/gobgp,a16/gobgp,osrg/gobgp,osrg/gobgp,fujita/gobgp,tamihiro/gobgp,fujita/gobgp