Make per-destination connection concurrency limit configurable - #8414

Summary

This PR makes ConnectionHandler's per-destination connection concurrency limit configurable via options, resolving #8322.

Background & Solution

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

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

Because open_connection() holds that semaphore slot for the connection's entire lifetime, high-concurrency environments or tools proxying multiple requests to a single destination address experienced blocking with no configuration option to raise the limit.

This change:

1. Adds connection_max_per_address (int, default 5) to Options in mitmproxy/options.py.
2. Updates ConnectionHandler.__init__ in mitmproxy/proxy/server.py to initialize self.max_conns using context.options.connection_max_per_address.
3. Adds unit test coverage in test/mitmproxy/test_options.py.

Fixes #8322