Allow connect-only connections to be closed when unused #5749
As mentioned in #4426 (comment), commit 1d5c427 fixes issue 4426 but introduces another issue where connect-only connections completed in a multi handle would be kept alive for the entire lifetime of that multi handle. This patch disassociates these connections from their easy handle when that easy handle is removed from the multi, allowing them to be cleaned up normally.
Previously any connect-only connections in a multi handle would be kept alive until the multi handle was closed. Since these connections cannot be re-used, they can be marked for closure when the associated easy handle is removed from the multi handle.