[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}

platform/default/default_file_source.cpp

@@ -19,15 +19,10 @@ namespace mbgl {
 
 class DefaultFileSource::Impl {
 public:
-    Impl(ActorRef<Impl> self, std::shared_ptr<FileSource> assetFileSource_, const std::string& cachePath, uint64_t maximumCacheSize)
+    Impl(std::shared_ptr<FileSource> assetFileSource_, std::string cachePath, uint64_t maximumCacheSize)
             : assetFileSource(assetFileSource_)
-            , localFileSource(std::make_unique<LocalFileSource>()) {
-        // Initialize the Database asynchronously so as to not block Actor creation.
-        self.invoke(&Impl::initializeOfflineDatabase, cachePath, maximumCacheSize);
-    }
-
-    void initializeOfflineDatabase(std::string cachePath, uint64_t maximumCacheSize) {
-        offlineDatabase = std::make_unique<OfflineDatabase>(cachePath, maximumCacheSize);
+            , localFileSource(std::make_unique<LocalFileSource>())
+            , offlineDatabase(std::make_unique<OfflineDatabase>(cachePath, maximumCacheSize)) {
     }
 
     void setAPIBaseURL(const std::string& url) {