MINOR: connection: perform the call to xprt->shutw() in conn_data_shutw()

MINOR: connection: perform the call to xprt->shutw() in conn_data_shutw()

This will save callers from having to care about conn->xprt and xprt->shutw.
Note that shutw() takes a second argument indicating whether it's a clean or
a hard shutw. This is used by SSL which tries to close cleanly in most cases.

Here we provide two versions, conn_data_shutw() which performs the clean
close, and conn_data_shutw_hard() which does the unclean one.