Extract pool package. Add pool benchmark.

internal/pool/pool_single.go

@@ -0,0 +1,47 @@
+package pool
+
+type SingleConnPool struct {
+	cn *Conn
+}
+
+func NewSingleConnPool(cn *Conn) *SingleConnPool {
+	return &SingleConnPool{
+		cn: cn,
+	}
+}
+
+func (p *SingleConnPool) First() *Conn {
+	return p.cn
+}
+
+func (p *SingleConnPool) Get() (*Conn, bool, error) {
+	return p.cn, false, nil
+}
+
+func (p *SingleConnPool) Put(cn *Conn) error {
+	if p.cn != cn {
+		panic("p.cn != cn")
+	}
+	return nil
+}
+
+func (p *SingleConnPool) Remove(cn *Conn, _ error) error {
+	if p.cn != cn {
+		panic("p.cn != cn")
+	}
+	return nil
+}
+
+func (p *SingleConnPool) Len() int {
+	return 1
+}
+
+func (p *SingleConnPool) FreeLen() int {
+	return 0
+}
+
+func (p *SingleConnPool) Stats() *PoolStats { return nil }
+
+func (p *SingleConnPool) Close() error {
+	return nil
+}