oxenstored: catch the error when a connection is already deleted

oxenstored: catch the error when a connection is already deleted

The function process_fdset_with is called on the read set connections first.
During the process, it might destroy a connection and remove it from the
connections database if some errors occur. However, a reference to the same
connection might still exist in the write set, which is awaiting to be
processed next. In this case, a Not_found error will be raised and the process
is aborted.

This patch changes the logic to ignore connections just missing from the
connection database and continue the rest part of the work.

Signed-off-by: Zheng Li <dev@zheng.li>
Reviewed-by: David Scott <dave.scott@citrix.com>