When the per-host connection cap is saturated and HTTP/2 is enabled, a request that fails to acquire a permit tries to multiplex onto a sibling connection another request is establishing to the same origin (stream reuse needs no permit). Off the event loop, waitForHttp2Connection did this by Thread.sleep(10)-polling the H2 registry until connectTimeout (5s default) elapsed — parking the caller thread (the synchronous part of execute()) for up to the full timeout and burning CPU. Under a bounded caller thread pool, a burst of over-cap requests to a new H2 origin could exhaust the pool.

Replace the busy-poll with an event-driven, non-blocking deferral: register a one-shot waiter keyed by the request's partition key and return the pending future immediately. registerHttp2Connection wakes matching waiters, which resume the send via sendRequestWithOpenChannel. A connectTimeout deadline on the Netty timer fails the request with the original permit exception if no connection arrives; the client-close path fails pending waiters (their request-timeout backstop is not scheduled yet). A once-only CAS makes the registration, deadline, and close paths mutually exclusive, and the waiter rechecks the registry after registering to close the poll-vs-register lost-wakeup race.

On the event loop (a redirect / 401 / 407 retry) the single immediate poll is kept and we give up if it misses — a wait there could self-deadlock, since the connection is being established on that same loop. The ROUND_ROBIN AsyncHttpClient#2214 limitation is unchanged (per-IP keying) but no longer occupies the caller thread.

Adds ChannelManagerHttp2WaiterTest covering wake-on-registration, key isolation, waiter removal, and fail-on-close. Existing HTTP/2 regression tests (conformance, multiplexing, stream-orphan, streaming-body flow-control) pass unchanged.