core: add multiple engine support

The skeleton for this is in place, but it needs to be implemented.

Most of the library has been designed with the assumption that the `EnvoyEngine` will be instantiated (potentially multiple times) within the process. However, the `Engine` itself currently is held in a static weak pointer. We have a method to return an engine identifier, that currently is stubbed to simply return `1`.

In an earlier change, I made this type pointer width, and my plan for when we fully switch over will be to actually store the memory location of the `Engine` as the identifier. (This id is opaque and never actually exposed above the bridge layer.)

Because of this, we won't actually need a collection to hold `Engine` instances. Instead we can create an `Engine` with `new`, and store it until the platform engine wrapper is deallocated. At this point, we can `drain` and then ultimately `delete` the `Engine` instance.

With https://github.com/lyft/envoy-mobile/pull/923 resolving the last major category of shutdown crashes, the door should be open to make this change. There are a handful of locations in code where we intentionally currently leak memory as a shortcut since the `Engine` itself has been static anyways. These are commented, and I'll subsequently update those comments with a reference to this issue so we can be sure to clean them up.