swim: do not rebuild packet meta multiple times

Before the patch there were 2 cases when an unchanged packet was
rebuilt partially on each send:

  - cached round message's meta section was rebuilt on each
    EV_WRITE event in swim_scheduler_on_output() function;

  - broadcast message's meta section was rebuilt too even though
    its content does not depend on a broadcast interface.

The third case appears with indirect pings patch which aggravates
meta building business by routing and packet forwarding. When a
packet needs to be forwarded farther, its meta is built in a
special manner preserving the route before EV_WRITE appears, and
on_output should not touch that meta.

This patch adds a check preventing unnecessary meta rebuilds.
Besides, the check and the meta building code are moved into a
dedicated function out of swim_scheduler_on_output() - it allows
to completely split logic of packing a message and sending it.
Separated logic helps a lot when indirect pings are introduced.

Part of #3234