vine: do not attempt expensive operations at every iteration of the wait loop

[btovar]

Both the check for fixed inputs and scheduling of replicas are potentially time consuming:

• They should not be attempted at every iteration. For example, they can be called only N number of seconds apart, or perhaps only once per vine_wait call.
• They should not go through all the possible queued items. For example, the scheduling of replicas should use HASH_TABLE_ITERATE_FROM_KEY and only try to schedule for M number of files.