server: Add threadlocal_get_conn

server: Add threadlocal_get_conn

We'd like to interrupt a sleep during a filter/plugin transaction if
it is obvious that finishing the transaction is pointless (because the
client has delivered NBD_CMD_DISC or hit EOF).  But to do that, we
need to get back at the connection object from a function called from
the plugin thread.  Passing the connection as an opaque object would
be a major API change, so it's easier to just add a thread-local
tracking the current connection.

Signed-off-by: Eric Blake <eblake@redhat.com>