Add deadline, timeout, and hot-path controls to the connection loop

[lpgauth]

Summary

Extends whitecap with the controls a latency-sensitive workload needs to honor a per-request deadline and stay resilient under load, plus two hot-path improvements. All new timeouts default to infinity and fall back to a set receive_timeout, so behaviour is unchanged unless configured.

• Per-request arrival timestamp. #whitecap_req{} gains a received field (os:system_time(), native units), stamped when the first byte arrives, so a handler can measure time already spent on the wire and shed against a deadline.
• Split idle vs. in-request timeouts. receive_timeout splits into keepalive_timeout (idle wait for the next request) and request_timeout (per-read wait once a request has started arriving). A short request_timeout gives slowloris protection without dropping healthy idle keep-alive connections. receive_timeout, if set, remains the back-compat default for both.
• Optional handler deadline. handler_timeout (default infinity). A finite value runs the handler in a monitored process; an overrun is killed and answered 504, emitting [whitecap, handler, timeout]. infinity keeps the handler inline with no per-request process — the right choice when the handler enforces its own deadline.
• Hoisted config. handler_timeout, keepalive_timeout, max_keepalive, and request_timeout are read once at connection start rather than on every request/read.
• Length-aware body reads. A body with a known Content-Length is read with an exact-length gen_tcp:recv, landing in one read and one concat instead of repeatedly re-buffering partial reads (previously O(n²) on a body fragmented across TCP segments).

Docs (README config + telemetry tables, received usage note) and CHANGELOG updated under ## Unreleased. No version bump.

Verified: strict compile (warnings-as-errors), xref, and dialyzer clean; eunit 32 passed, including 4 new tests covering the arrival timestamp, split-body reassembly, 504 on handler overrun (~100ms), and 408 on a stalled request (~300ms).