uwt: switch to close_wait from close
uwt: switch to close_wait from close

The `close` function performs some cleanups asynchronously. This
patch switches to `close_wait` to minimise the chance we run out
of resources if we constantly open and close connections.

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@docker.com>

djs55/vpnkit,djs55/vpnkit,djs55/vpnkit