In the earlier http2 package migration (1d93bdce20ddd2, #17394) I had
removed Direct.Close's tracking of the connPool, thinking it wasn't
necessary.
Some tests (in another repo) are strict and like it to tear down the
world and wait, to check for leaked goroutines. And they caught this
letting some goroutines idle past Close, even if they'd eventually
close down on their own.
This restores the connPool accounting and the aggressife close.
Updates #17305
Updates #17394
Change-Id: I5fed283a179ff7c3e2be104836bbe58b05130cc7
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
206d98e84b