ds: support enqueued CODE refs with arguments

ds: support enqueued CODE refs with arguments

While we inherited support for enqueuing CODE refs from
Danga::Socket, it makes sense to support enqueuing CODE
refs with extra arguments specific to that CODE ref[1]
instead of relying on anonymous CODE refs to capture its
args..

Supporting args with CODE refs allows our ->flush_write
implementation to be more generic and no longer specific to
tmpio write buffering.  This change may eventually allow us
to chain multiple long_response calls together and eliminate
the {long_cb} field.

[1] we prefer CODE refs point to named subs with arguments
    passed to it to save RAM as opposed to constantly allocating
    new anonymous subs to capture local variables.