fix incompatibility with express-async-errors

[rochdev]

This PR fixes an incompatibility with express-async-errors. The module defines a getter/setter on Layer.prototype.handle where it wraps the layer handler when it gets set. The problem is that we overwrite the handler in the express instrumentation, but after it's already been wrapped by the module. This means that we wrap their wrapper, and then reassign layer.handle which causes the setter to wrap our wrapper. This end up with a situation where we then have asyncWrapper -> ddWrapper -> asyncWrapper -> handler which results in unexpected behaviors.

Since express-async-errors adds a __handle property to the layer, we can patch that instead without triggering the setter.

The behavior is different depending on the version of Node and in some cases internal to Express, so I was unable to write a proper unit test for this.

Fixes #548