[Miniflare 3] Re-enable concurrent `dispatchFetch()`s and batch proxy heap frees

[mrbbot]

In order to fix tests when adding the magic proxy, we restricted dispatchFetch() to one concurrent TCP connection. Unfortunately, this prevented long-lived dispatchFetch() requests.

When proxies are garbage collected, we send a network request to workerd to free the corresponding entry in the ProxyServer heap. Garbage collection often happens in phases though, freeing lots of objects at once. This caused many concurrent requests to workerd (~60 in some tests), leading to many TCP connections being created.

This change re-enables using multiple TCP connections, and attempts to address the root cause of the issues we were seeing, by batching frees before sending the network request.

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Latest commit: 6124292c7ec4b0ebc076b515d26ed656e8ed83d0

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