lease: use monotimer for lease

lease uses monotimer to calculate its expiration. In this way, changing system time won't affect in lease expiration.

FIX #6700

build

@@ -32,11 +32,13 @@ toggle_failpoints() {
 
 etcd_build() {
 	out="bin"
+	on=0
+	if [[ "$OSTYPE" == "darwin"* ]]; then on=1; fi
 	if [ -n "${BINDIR}" ]; then out="${BINDIR}"; fi
 	toggle_failpoints
 	# Static compilation is useful when etcd is run in a container
-	CGO_ENABLED=0 go build $GO_BUILD_FLAGS -installsuffix cgo -ldflags "$GO_LDFLAGS" -o ${out}/etcd ${REPO_PATH}/cmd/etcd || return
-	CGO_ENABLED=0 go build $GO_BUILD_FLAGS -installsuffix cgo -ldflags "$GO_LDFLAGS" -o ${out}/etcdctl ${REPO_PATH}/cmd/etcdctl || return
+	CGO_ENABLED=$on go build $GO_BUILD_FLAGS -installsuffix cgo -ldflags "$GO_LDFLAGS" -o ${out}/etcd ${REPO_PATH}/cmd/etcd || return
+	CGO_ENABLED=$on go build $GO_BUILD_FLAGS -installsuffix cgo -ldflags "$GO_LDFLAGS" -o ${out}/etcdctl ${REPO_PATH}/cmd/etcdctl || return
 }
 
 etcd_setup_gopath() {