ConnectionHandler's per-destination connection concurrency (asyncio.Semaphore(5)) is hardcoded with no way to configure it #8322

Problem

ConnectionHandler.__init__ (mitmproxy/proxy/server.py) hardcodes a concurrency limit of 5 open connections per destination address:

self.max_conns = collections.defaultdict(lambda: asyncio.Semaphore(5))

open_connection() holds that semaphore slot for the connection's entire lifetime, not just the handshake:

async with self.max_conns[command.connection.address]:
    ...
    try:
        await self.handle_connection(command.connection)
    finally:
        ...

So a 6th concurrent connection to the same destination address blocks until one of the existing 5 fully closes — and there's no Options field, --set flag, or any other configuration surface for this value; it's a bare literal with zero visibility (confirmed against current main).

This is a real problem for anything that legitimately needs many concurrent connections to one host — e.g. a browser client that multiplexes many requests to one origin over HTTP/2, which mitmproxy re-originates as separate HTTP/1.1 connections upstream (http2=false on the outbound leg), or any tool proxying high-concurrency traffic toward a single target. Related, though it never landed on this specific mechanism: #4451 (closed 2021) discusses similarly-shaped "requests hang for a long time" symptoms without conclusively tying it to this cap.

Proposed fix

Make the cap a configurable Options field, defaulting to 5 to preserve current behavior:

# mitmproxy/options.py, near tcp_timeout
self.add_option(
    "connection_max_per_address",
    int,
    5,
    """
    Maximum number of concurrent connections mitmproxy will hold open to any single
    destination address. A destination that already has this many connections open
    will delay new ones until one of the existing connections closes.
    """,
)

# mitmproxy/proxy/server.py, ConnectionHandler.__init__
self.max_conns = collections.defaultdict(
    lambda: asyncio.Semaphore(context.options.connection_max_per_address)
)

Happy to open a PR with this exact change (plus a test) if the shape looks right — opening this issue first per CONTRIBUTING.md's guidance to talk before a larger change, though this one is small.