Add max_requests client option

[lpgauth]

Summary

Adds a max_requests client option (pos_integer() | infinity, default infinity) that limits how many requests a connection handles before it is recycled, as requested in #69.

When the cap is reached, the server is disabled in the pool so no new requests are routed to it, in-flight requests drain normally, then the socket is closed and reconnected immediately (skipping the reconnect backoff, since the connection was healthy). Reconnecting goes through inet:getaddrs/2, so DNS is re-resolved on every recycle — enabling periodic rebalancing across DNS-based load balancers.

• Recycling is checked after each send, after processing replies, and after sweep expiry, so the drain converges even for noop-only or timed-out tails.
• requests_left is reset on every successful connect; error-driven reconnects are unaffected.
• Documented in the README client_options table and covered by a new eunit test that asserts the socket is replaced after max_requests calls while requests keep succeeding.

Closes #69