Pave more road for prefetch:

Pave more road for prefetch:

Sanity-check values assigned to obj.prefetch from VCL: if before present
time or after ttl, SHM a VCL_info message and set obj.prefetch to zero
disabling prefetch.

Change objects/timer interaction:  We only keep track of the first timer for
each object (->timer_when) and add a field to remember what we intend to
do at that time (->timer_what).

Rename heap_idx to timer_idx to group the relevant fields (XXX: it should be
accessed through function outside cache_expire.c which should be called
cache_timer.c now).

Abolish the 30 second advance move to death-row.

When the prefetch timer expires, SHM a debug message and don't do anything.

Minor polishing and cleanup.

Add vcl_prefetch{} to default VCL and set prefetch 30 seconds before ttl.

git-svn-id: svn+ssh://projects.linpro.no/svn/varnish/trunk@2344 d4fa192b-c00b-0410-8231-f00ffab90ce4