Add finalizer to Response and use an pooled arena

Unlike XHR, Response is a bit more complicated as it can exist in Zig code
without ever being given to v8. So we need to track this handoff to know who is
responsible for freeing it (zig code, on error/shutdown) or v8 code after
promise resolution.

This also cleansup a bad merge for the XHR finalizer and adds cleaning up the
`XMLHttpRequestEventTarget` callbacks.

src/http/Client.zig

@@ -186,7 +186,7 @@ pub fn abort(self: *Client) void {
     while (n) |node| {
         n = node.next;
         const transfer: *Transfer = @fieldParentPtr("_node", node);
-        self.transfer_pool.destroy(transfer);
+        transfer.kill();
     }
     self.queue = .{};
 
@@ -392,6 +392,8 @@ fn requestFailed(self: *Client, transfer: *Transfer, err: anyerror, comptime exe
 
     if (execute_callback) {
         transfer.req.error_callback(transfer.ctx, err);
+    } else if (transfer.req.shutdown_callback) |cb| {
+        cb(transfer.ctx);
     }
 }
 
@@ -781,6 +783,7 @@ pub const Request = struct {
     data_callback: *const fn (transfer: *Transfer, data: []const u8) anyerror!void,
     done_callback: *const fn (ctx: *anyopaque) anyerror!void,
     error_callback: *const fn (ctx: *anyopaque, err: anyerror) void,
+    shutdown_callback: ?*const fn (ctx: *anyopaque) void = null,
 
     const ResourceType = enum {
         document,
@@ -995,6 +998,9 @@ pub const Transfer = struct {
         if (self._handle != null) {
             self.client.endTransfer(self);
         }
+        if (self.req.shutdown_callback) |cb| {
+            cb(self.ctx);
+        }
         self.deinit();
     }