refactor: remove unnecessary xsync.Map wrapper

- Updated agentPool, cachedAddr, fileContentMap, and lastSeenMap to use xsync.Map.
- Removed functional package and its related tests.

agent/pkg/agent/agent_pool.go

@@ -3,11 +3,11 @@ package agent
 import (
 	"iter"
 
+	"github.com/puzpuzpuz/xsync/v4"
 	"github.com/yusing/go-proxy/internal/common"
-	"github.com/yusing/go-proxy/internal/utils/functional"
 )
 
-var agentPool = functional.NewMapOf[string, *AgentConfig]()
+var agentPool = xsync.NewMap[string, *AgentConfig](xsync.WithPresize(10))
 
 func init() {
 	if common.IsTest {