Dynamic flowtable entry allocation

Reviewer: @dtalayco

The goal is to reduce the memory usage of IVS, which has no hard limit on the maximum number of flows. Setting a reasonable upper bound on the number of flows in IVS would result in hundreds of megabytes of allocated memory using the existing statically allocated flowtable.

This pull request also contains a bunch of simplification and dead code removal commits.

The new flowtable iterator is necessary because the array of flowtable entries is removed, which was used by ind_core_spawn_ft_iter.

I also plan to dynamically grow the hashtable bucket arrays in a later pull request.