[Mootjelh]

Fixes bogdanfinn/tls-client#289, along the lines michal-billtech proposed there.

A request with Connection: close gets a single-use connection. idleStateLocked counted that connection as spent once nextStreamID > 1, which assumes the first stream is 1. In fhttp it is not always: InitialStreamID, AllowHTTP and Priorities all move nextStreamID in newClientConn, before any request. So a fresh single-use connection reported that it could not take a request, RoundTrip got errClientConnUnusable, and the transport dialed a new connection, again and again. Firefox profiles start at stream 3, so this hits every one of them.

newClientConn now records where the connection starts (firstStreamID), and both checks in idleStateLocked compare against that instead of 1.

TestSingleUseConnServesItsRequestWhenTheFirstStreamIsNotOne runs four cases against a local HTTP/2 server and counts the connections the server sees:

• first stream 1 with Connection: close, and InitialStreamID 3 without it. Both pass on master too, they are the controls
• InitialStreamID 3 with Connection: close. On master: no response after 5s, 4 connections dialed
• Priorities with Connection: close. Same on master

Each case must get a response over exactly one connection. Reverting either the comparison or the assignment fails the test.

The freshConn change only matters for the GetConn trace hook when the transport comes from ConfigureTransport. The test does not cover it, but it rests on the same assumption, so I changed it too.

go test ./... on Windows: the same 8 failures on master and on this branch (ServeFile, ServeContent, OmitHTTP2, cgi DirWindows), and none of them are in http2. ./http2 passes, also with -race.

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