Issue #4692 - Rewrite I/O tasks to avoid engine self-reference

Issue #4692 - Rewrite I/O tasks to avoid engine self-reference

Rewrite I/O callbacks and tasks to use native v8 types instead of the wrapped
one with embedded shared pointers to the engine.

Keep a single `JsEnginePtr` in each I/O task object. This compensates for the
deficiency that we have no other way of keeping an engine in existence for the
duration of the task. Improve the consistency and breadth of argument
validation for the callback functions implementing the I/O API.

Eliminate base class `IoTask`. It wasn't doing anything important enough to
merit its continued existence.

Move initialization of global object into constructor of `JsEngineInternal`.

Added utility function `JsEngineInternal::ToV8String()`. This allows the
elimination of a lot of calls to `GetIsolate()`.

[Eric, 2016-12-28 17:46:05]

This is the last of the task rewrites to get rid of almost all the engine
self-reference.