Lock-free thread worker event loop with exhaustion protection and backpressure

Replace RLock-based synchronization with PollableMethodQueue (pipe-based
method deferral) so all poller and keepalive operations run on the main
thread without locks. Unified single poller.select() loop replaces the
dual-path poller.select() + futures.wait() pattern.

Add settimeout(KEEPALIVE) on new connections to prevent slow clients from
holding thread pool slots indefinitely. Add set_accept_enabled() to stop
accepting connections when at WORKER_CONNECTIONS capacity.

Author: davegaeddert