Turn the proxy code into a library

Allow other go programs to easily import the snowflake proxy library and
start/stop a snowflake proxy.

proxy/lib/tokens.go

@@ -0,0 +1,44 @@
+package snowflake
+
+import (
+	"sync/atomic"
+)
+
+type tokens_t struct {
+	ch       chan struct{}
+	capacity uint
+	clients  int64
+}
+
+func newTokens(capacity uint) *tokens_t {
+	var ch chan struct{}
+	if capacity != 0 {
+		ch = make(chan struct{}, capacity)
+	}
+
+	return &tokens_t{
+		ch:       ch,
+		capacity: capacity,
+		clients:  0,
+	}
+}
+
+func (t *tokens_t) get() {
+	atomic.AddInt64(&t.clients, 1)
+
+	if t.capacity != 0 {
+		t.ch <- struct{}{}
+	}
+}
+
+func (t *tokens_t) ret() {
+	atomic.AddInt64(&t.clients, -1)
+
+	if t.capacity != 0 {
+		<-t.ch
+	}
+}
+
+func (t tokens_t) count() int64 {
+	return atomic.LoadInt64(&t.clients)
+}