Add max_connections config (default 256) with -m/--max-connections CLI
flag. Server wraps on_client in asyncio.Semaphore to prevent fd
exhaustion under load. Value reloads on SIGHUP; active connections
drain normally. Also adds pool_size/pool_max_idle config fields and
first_hop_pool wiring in server.py (used by next commits), and fixes
asyncio.TimeoutError -> TimeoutError lint warnings.
Asyncio-based SOCKS5 server that tunnels connections through
configurable chains of SOCKS5, SOCKS4/4a, and HTTP CONNECT proxies.
Tor integration via standard SOCKS5 hop.