Only crash in debug mode, if HTTPClient was not shutdown (#478)

### Motivation

Generally we want to inform users that they need to shutdown their HTTPClient. Until `1.6.0` we did this with an assert in HTTPClient's deinit. With `1.6.0` this behavior was raised to a precondition. Because of this adopters might suddenly crash in production where they didn't before.

### Changes

- This pr reverts the current behavior back to something pre `1.6.0`

### Result

- HTTPClient doesn't crash in production anymore.

Sources/AsyncHTTPClient/HTTPClient.swift

@@ -68,7 +68,7 @@ public class HTTPClient {
     let eventLoopGroupProvider: EventLoopGroupProvider
     let configuration: Configuration
     let poolManager: HTTPConnectionPool.Manager
-    var state: State
+    private var state: State
     private let stateLock = Lock()
 
     internal static let loggingDisabled = Logger(label: "AHC-do-not-log", factory: { _ in SwiftLogNoOpLogHandler() })
@@ -118,8 +118,22 @@ public class HTTPClient {
     }
 
     deinit {
-        guard case .shutDown = self.state else {
-            preconditionFailure("Client not shut down before the deinit. Please call client.syncShutdown() when no longer needed.")
+        debugOnly {
+            // We want to crash only in debug mode.
+            switch self.state {
+            case .shutDown:
+                break
+            case .shuttingDown:
+                preconditionFailure("""
+                This state should be totally unreachable. While the HTTPClient is shutting down a \
+                reference cycle should exist, that prevents it from deinit.
+                """)
+            case .upAndRunning:
+                preconditionFailure("""
+                Client not shut down before the deinit. Please call client.syncShutdown() when no \
+                longer needed. Otherwise memory will leak.
+                """)
+            }
         }
     }
 

Sources/AsyncHTTPClient/Utils.swift

@@ -32,3 +32,13 @@ public final class HTTPClientCopyingDelegate: HTTPClientResponseDelegate {
         return ()
     }
 }
+
+/// A utility function that runs the body code only in debug builds, without
+/// emitting compiler warnings.
+///
+/// This is currently the only way to do this in Swift: see
+/// https://forums.swift.org/t/support-debug-only-code/11037 for a discussion.
+@inlinable
+internal func debugOnly(_ body: () -> Void) {
+    assert({ body(); return true }())
+}