Implement ActorSingleton for DistributedActor (#980)

* Implement ActorSingleton for DistributedActor

Resolves #824

* Fix compilation errors

* Rename and better APIs

* Update docs

* Formatting; revert unneeded changes

* Only singleton needs to have known name

* Implement ActorSingleton for DistributedActor

Resolves #824

* rework how we get hold of intercepted actors

* rework how we get hold of intercepted actors

* Rebase and move ClusterSingletonPlugin into DistributedActors module

* Move singleton tests

* Singleton allocation should be time bound

* Remove unnecessary actor

* Add test for allocation timeout

* Respect buffer capacity

* clean up FIXMEs

* Make plugin shutdown async

Co-authored-by: Konrad `ktoso` Malawski <[email protected]>

Author: yim-lee

Package.swift

@@ -34,16 +34,6 @@ var targets: [PackageDescription.Target] = [
         ]
     ),
 
-    // ==== ------------------------------------------------------------------------------------------------------------
-    // MARK: Plugins // TODO: rename since may be confused with package plugins?
-
-    .target(
-        name: "ActorSingletonPlugin",
-        dependencies: [
-            "DistributedActors",
-        ]
-    ),
-
     .executableTarget(
         name: "ExecApp",
         dependencies: [
@@ -71,7 +61,6 @@ var targets: [PackageDescription.Target] = [
 //        name: "DistributedActorsDocumentationTests",
 //        dependencies: [
 //            "DistributedActors",
-//            "ActorSingletonPlugin",
 //            "DistributedActorsTestKit",
 //        ],
 //        exclude: [
@@ -105,14 +94,6 @@ var targets: [PackageDescription.Target] = [
 //            "CDistributedActorsMailbox",
 //            "DistributedActorsTestKit"
 //        ]
-//    ),
-//
-//    .testTarget(
-//        name: "ActorSingletonPluginTests",
-//        dependencies: [
-//            "ActorSingletonPlugin",
-//            "DistributedActorsTestKit"
-//        ]
 //    ),
 
     // ==== ------------------------------------------------------------------------------------------------------------
@@ -219,13 +200,6 @@ let products: [PackageDescription.Product] = [
             "ExecApp",
         ]
     ),
-
-    /* --- Functional Plugins --- */
-
-    .library(
-        name: "ActorSingletonPlugin",
-        targets: ["ActorSingletonPlugin"]
-    ),
 ]
 
 // This is a workaround since current published nightly docker images don't have the latest Swift availabilities yet