Honour HeaderReceived's ReadTimeout per HTTP/2 stream

[localrivet]

EdgeLB gives one route, `POST /api/v1/files/upload`, a 10-minute read timeout through `fasthttp.Server.HeaderReceived`. Over HTTP/1.1 fasthttp applies that itself; over HTTP/2 this fork gave every stream the server-wide `ReadTimeout` (120 s at the edge), so a slow phone upload over HTTP/2 was still cut at 120 s.

Change

• When a stream's headers end, the server calls `HeaderReceived`; a non-zero `ReadTimeout` replaces the server's for that one stream, measured from the stream's start.
• The request timer arms for the earliest deadline among streams still receiving their request, and re-arms on every new stream (a long deadline must not hold the timer past a shorter one). The timeout sweep checks every stream instead of stopping at the first not-due one, since deadlines are no longer in creation order. With no hook set, behaviour is unchanged.
• Only `ReadTimeout` is honoured; `MaxRequestBodySize` and `WriteTimeout` from the hook are not applied per stream (documented on the field).

Tests

• New `TestHeaderReceivedReadTimeoutPerStream`: server ReadTimeout 300 ms; a ~1 s upload on the path the hook extends to 10 s completes with 200, the same upload on another path is cut.
• `go test ./` passes.
• `go test ./h2spec`: the same four cases fail on master without this change (4.2/2, 4.2/3, 5.1/8, 5.1/11), and 5.1/5, 5.1.1/2, 6.1/2 flake on both master and this branch over 12 runs each. Not caused by this change.

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