Add VASYNC_MODE=direct setting

[j3k0]

Some mockup libraries (like testdouble) makes it easy to turn synchronous code into synchronous code (for easier testing).

By default, vasync breaks synchronous flow by calling callbacks inside a setImmediate block.

While this is desirable in production (generally also when running tests), it is also useful to let the user disable this behavior when required.

[j3k0]

Note, a side effect of this PR is also to stop defining setImmediate() globally when it's not already defined.