thread safe connection pool (#40)

* thread safe connection pool

* coarse grain locking + event loop preference

* use embedded loop for pooling test

* rm atomic + use circular buffer for available

* use gh actions for ci

* add logging

* el preference docs + use while loop for prune

* add delgation test

.github/FUNDING.yml

@@ -0,0 +1,2 @@
+github: [tanner0101] # loganwright, joscdk
+open_collective: vapor

.github/workflows/test.yml

@@ -0,0 +1,25 @@
+name: test
+on:
+- pull_request
+jobs:
+  xenial:
+    container: 
+      image: swift:5.1-xenial
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@master
+    - run: swift test
+  bionic:
+    container: 
+      image: swift:5.1-bionic
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@master
+    - run: swift test
+  thread:
+    container: 
+      image: swift:5.1-bionic
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@master
+    - run: swift test --sanitize=thread

Package.swift

@@ -8,9 +8,10 @@ let package = Package(
     ],
     dependencies: [
         .package(url: "https://github.com/apple/swift-nio.git", from: "2.0.0"),
+        .package(url: "https://github.com/apple/swift-log.git", from: "1.0.0"),
     ],
     targets: [
-        .target(name: "AsyncKit", dependencies: ["NIO"]),
+        .target(name: "AsyncKit", dependencies: ["Logging", "NIO"]),
         .testTarget(name: "AsyncKitTests", dependencies: ["AsyncKit"]),
     ]
 )