SPDY: avoid setting timeout on stream events in ngx_http_writer().

The SPDY module doesn't expect timers can be set on stream events for reasons
other than delaying output.  But ngx_http_writer() could add timer on write
event if the delayed flag wasn't set and nginx is waiting for AIO completion.
That could cause delays in sending response over SPDY when file AIO was used.
SPDY: added a comment about handling stream with the timer set.
Variables: fixed non-indexed access of prefix vars (ticket #600).

Previously, a configuration like

    location / {
        ssi on;
        ssi_types *;
        set $http_foo "bar";
        return 200 '<!--#echo var="http_foo" -->\n';
    }

resulted in NULL pointer dereference in ngx_http_get_variable() as
the variable was explicitly added to the variables hash, but its
get_handler wasn't properly set in the hash.  Fix is to make sure
that get_handler is properly set by ngx_http_variables_init_vars().
Variables: updated list of prefixes in ngx_http_rewrite_set().