net/http: remove persistConn reference from wantConn

[AlexanderYastrebov]

Transport getConn creates wantConn w, tries to obtain idle connection for it
based on the w.key and, when there is no idle connection, puts wantConn into
idleConnWait wantConnQueue.

Then getConn dials connection for w in a goroutine and blocks.
After dial succeeds getConn unblocks and returns connection to the caller.

At this point w is stored in the idleConnWait and will not be evicted
until another wantConn with the same w.key is requested or alive
connection returned into the idle pool which may not happen e.g. if
server closes the connection.

The problem is that even after tryDeliver succeeds w references
persistConn wrapper that allocates bufio.Reader and bufio.Writer and
prevents them from being garbage collected.

To fix the problem this change removes persistConn and error references
from wantConn and delivers them via channel to getConn.

This way wantConn could be kept in wantConnQueues arbitrary long.

Fixes #43966
Fixes #50798

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Patch Set 1:

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Patch Set 2:

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Patch Set 2: Commit-Queue+1

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FAILURE

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Error:
failed to run "test golang.org/x/tools module": exit status 1
failed to run "test golang.org/x/tools/gopls module": exit status 1

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• "test golang.org/x/tools module" (combined output)
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This PR is being closed because golang.org/cl/522095 has been merged.