[core] Avoid blocking in Thread<Object> constructor (#12151)

* Introduce AspiringActor, EstablishedActor

This pair of objects represents the two-phase (parent-thread /
child-thread) construction that's needed to support constructing
Thread<Object> without blocking until the child thread is up and
running.

An `AspiringActor<O>` is responsible for:
 - ownership of the actor's `Mailbox`
 - allocating the memory for (but *not* constructing) the target object `O`

Using these two pieces--the mailbox and a stable address for `O`--an
`AspiringActor<O>` can accept messages for the target object, or provide
`ActorRef<O>`s that do so, before the object has actually been
constructed by the corresponding `EstablishedActor<O>`.  (Such messages
are queued in the mailbox until after the object is constructed.)

This allows for an `AspiringActor<O>` to be created and safely used by a
thread other than the one on which the target object will (eventually)
live.

An `EstablishedActor<O>` is responsible for managing the lifetime of the
target object `O` and the open/closed state of the parent's `mailbox`.

The `O` object's lifetime is contained by that of its owning
`EstablishedActor<O>`: the `EstablishedActor` constructor executes the
`O` constructor via "placement new", constructing it at the address
provided by the parent `AspiringActor`, and the `~EstablishedActor`
destructor similarly executes the `~O` destructor (after closing the
mailbox). `EstablishedActor` should therefore live entirely on the
thread intended to own `O`.

* Remove Actor#{invoke,ask}

src/mbgl/tile/raster_dem_tile.cpp

@@ -48,7 +48,7 @@ void RasterDEMTile::setMetadata(optional<Timestamp> modified_, optional<Timestam
 void RasterDEMTile::setData(std::shared_ptr<const std::string> data) {
     pending = true;
     ++correlationID;
-    worker.invoke(&RasterDEMTileWorker::parse, data, correlationID, encoding);
+    worker.self().invoke(&RasterDEMTileWorker::parse, data, correlationID, encoding);
 }
 
 void RasterDEMTile::onParsed(std::unique_ptr<HillshadeBucket> result, const uint64_t resultCorrelationID) {