ContextWithTimeout middleware feature request

[hakankutluay]

Issue Description

Need timeout support on long-running operations like Db, HTTP calls, etc. timeout middleware supports cancellation with context but has race conditions.

We need a middleware that creates context with timeout and injects ContextWithTimeout to c.Request().Context()
New approach also fixes #2306.

Expected behaviour

We need to introduce a new middleware (middleware.ContextTimeout()) that creates context with timeout and injects ContextWithTimeout to c.Request().Context(). If the handler returns an error that wraps context.DeadlineExceeded, it returns Service Unavailable (503)

New approach also fixes #2306.

Actual behaviour

With current timeout middleware, since it has a race condition, it is not safe to use in production.

[vigo]

great work @hakankutluay ! thanx buddy!