优化内容压缩

* 取消用户设置的压缩级别，现在压缩级别通过系统自动设置
* Pool中的对象命中100万次时自动销毁，避免内存泄漏
* 降低Pool中的对象数量，避免占用太多内存
* 根据系统CPU线程数自动计算压缩级别，避免消耗太多CPU
* zstd限制解码的最大Window
* zstd使用低内存模式

internal/compressions/writer_base.go

@@ -2,10 +2,16 @@
 
 package compressions
 
+import (
+	"sync/atomic"
+)
+
 type BaseWriter struct {
 	pool *WriterPool
 
 	isFinished bool
+
+	hits uint32
 }
 
 func (this *BaseWriter) SetPool(pool *WriterPool) {
@@ -13,8 +19,11 @@ func (this *BaseWriter) SetPool(pool *WriterPool) {
 }
 
 func (this *BaseWriter) Finish(obj Writer) error {
+	if this.isFinished {
+		return nil
+	}
 	err := obj.RawClose()
-	if err == nil && this.pool != nil && !this.isFinished {
+	if err == nil && this.pool != nil {
 		this.pool.Put(obj)
 	}
 	this.isFinished = true
@@ -24,3 +33,7 @@ func (this *BaseWriter) Finish(obj Writer) error {
 func (this *BaseWriter) ResetFinish() {
 	this.isFinished = false
 }
+
+func (this *BaseWriter) IncreaseHit() uint32 {
+	return atomic.AddUint32(&this.hits, 1)
+}