Better to keep context.WithValue constant

[polyrabbit]

In stdlib, context.WithValue returns a derived context that points to the parent Context, and the parent Context is not modified.

In JuiceFS, context.WithValue mutates itself by stacking the Context field. This is error-prone as no one can clean the stack chain.

Especially in the gateway scenario where meta.Context is a global variable - its value chain can grow infinitely as gateway runs long enough. This causes a memory leak and makes searching for a non-existent key very expensive.

It's better to follow best practices in the Go community.