fix: concurrent write to websocket connection panic

[kooksee]

goroutine 680471 [running]:
github.com/gorilla/websocket.(*messageWriter).flushFrame(0xc00303d2c0, 0x1, {0xc004a5aabd?, 0x1?, 0xc00303d2f0?})
        /gocache/gomod/github.com/gorilla/[email protected]/conn.go:617 +0x4b8
github.com/gorilla/websocket.(*Conn).WriteMessage(0xc001a5e160, 0x476c073?, {0xc004a5a9b2, 0x10b, 0x164e})
        /gocache/gomod/github.com/gorilla/[email protected]/conn.go:770 +0x126
github.com/pubgo/lava/pkg/wsproxy.(*Proxy).proxy(0xc002449720, {0x555e690, 0xc003dbe680}, 0xc0018010e0)
        /gocache/gomod/github.com/pubgo/[email protected]/pkg/wsproxy/websocket_proxy.go:316 +0x11c5
github.com/pubgo/lava/pkg/wsproxy.(*Proxy).ServeHTTP(0xc002449720, {0x555e690, 0xc003dbe680}, 0xc0018010e0)
        /gocache/gomod/github.com/pubgo/[email protected]/pkg/wsproxy/websocket_proxy.go:66 +0x47
github.com/pubgo/lava/servers/grpcs.(*serviceImpl).DixInject.StripPrefix.func33({0x555e690, 0xc003dbe680}, 0xc001800fc0)
        /usr/local/go/src/net/http/server.go:2214 +0x262
net/http.HandlerFunc.ServeHTTP(0xc00141c008?, {0x555e690?, 0xc003dbe680?}, 0x18?)
        /usr/local/go/src/net/http/server.go:2171 +0x29
github.com/pubgo/lava/pkg/httputil.NewFastHTTPHandler.func1(0xc00141c008)
        /gocache/gomod/github.com/pubgo/[email protected]/pkg/httputil/fiber.go:89 +0x256
github.com/pubgo/lava/servers/grpcs.(*serviceImpl).DixInject.HTTPHandler.func34(0xc0029b4608)
        /gocache/gomod/github.com/pubgo/[email protected]/pkg/httputil/fiber.go:33 +0x3e
github.com/gofiber/fiber/v2.(*App).next(0xc001c9e008, 0xc0029b4608)
        /gocache/gomod/github.com/gofiber/fiber/[email protected]/router.go:145 +0x1be
github.com/gofiber/fiber/v2.(*App).handler(0xc001c9e008, 0x49e2ef?)
        /gocache/gomod/github.com/gofiber/fiber/[email protected]/router.go:172 +0x78
github.com/valyala/fasthttp.(*Server).serveConn(0xc001c98488, {0x5575930, 0xc00175b558})
        /gocache/gomod/github.com/valyala/[email protected]/server.go:2374 +0xe70
github.com/valyala/fasthttp.(*workerPool).workerFunc(0xc00259be00, 0xc0042ba2a0)
        /gocache/gomod/github.com/valyala/[email protected]/workerpool.go:224 +0xa4
github.com/valyala/fasthttp.(*workerPool).getCh.func1()
        /gocache/gomod/github.com/valyala/[email protected]/workerpool.go:196 +0x32
created by github.com/valyala/fasthttp.(*workerPool).getCh in goroutine 326
        /gocache/gomod/github.com/valyala/[email protected]/workerpool.go:195 +0x190