Move setting the sock->write_timeout to the async_*send

Setting the sock->write_timeout from the TCP, TCPDNS, and TLSDNS send
functions could lead to (harmless) data race when setting the value for
the first time when the isc_nm_send() function would be called from
thread not-matching the socket we are sending to.  Move the setting the
sock->write_timeout to the matching async function which is always
called from the matching thread.
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