[Varnish] #1075: Allow directors (specifically fallback director) to take other directors as backends

[Varnish]

#1075: Allow directors (specifically fallback director) to take other directors as
backends
----------------------------+-----------------------------------------------
 Reporter:  halcyonCorsair  |        Type:  enhancement
   Status:  new             |    Priority:  normal     
Milestone:                  |   Component:  build      
  Version:  3.0.2           |    Severity:  normal     
 Keywords:                  |  
----------------------------+-----------------------------------------------
 Hi,

 I'd like to be able to pass a director to another director as a backend,
 for example, using the fallback director to take 2 servers in a round-
 robin director before falling back to a 3rd servers.

 eg.

 {{{
 director frontend round-robin {
   { .backend = web1; }
   { .backend = web2; }
 }

 director default fallback {
   { .backend = frontend; }
   { .backend = web3; } // will only be used if servers in 'frontend'
 director are unhealthy.
 }
 }}}

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Ticket URL: <https://www.varnish-cache.org/trac/ticket/1075>
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