Regtest m b f changes reorg persistence

Note: this is a messy commit, parts of this could be broken up into several PRs instead of a big one.

This adds:

*deletion of saved SPs that are no longer in the activeChain (longest work chain) on process start (init in load_most_relevant_state())
*deletion of saved MP txes that are persisted in MP_txlist on the disconnect (pop) of a block due to being stale
*removal of persistence files associated with stale blockhashes (MP_persist)

note that the client MUST restart if they are to get the best state after a block is orphaned. Persistence is disabled after a block is orphaned, to prevent bad states from being saved to disk ('safe-mode').

In addition this includes:

*Regtest mode 
*3 test cases used to test the functionality for above
     -reorg_* basic tests for various reorg scenarios
     -fork.sh starts individual regtest nodes