Reuse push connections pinned to the address the guard just approved

Since push delivery was pinned to the IP resolved and guarded for it,
every push opens a new TCP and TLS connection: Net::HTTP::Persistent
looks the host up itself and can't be pinned. The handshake is one or
two extra round trips for every push.

WebPush::Connections keeps the pinned connections open for 30 seconds
and hands one out again only to a delivery whose own, fresh resolution
returned the same address for the same host. Net::HTTP only ever
reconnects to that address, so no request goes to an address the guard
didn't just approve. A connection the push service closed while idle is
replaced before the push is written; once it's written, a dropped
connection raises ConnectionLost instead of sending the push twice or
invalidating the subscription. A delivery without a resolved IP is no
longer sent at all, and net-http-persistent goes.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0142qgjggdJ2KDdGk7RF9Xm9

lib/web_push/pool.rb

@@ -5,7 +5,7 @@ class WebPush::Pool
   def initialize(invalid_subscription_handler:)
     @delivery_pool = Concurrent::ThreadPoolExecutor.new(max_threads: 50, max_queue: 10000)
     @invalidation_pool = Concurrent::FixedThreadPool.new(1)
-    @connection = Net::HTTP::Persistent.new(name: "web_push", pool_size: 150)
+    @connection = WebPush::Connections.new
     @invalid_subscription_handler = invalid_subscription_handler
   end