Outputs where all amounts are known spent can now be pruned

Only for pre rct for obvious reasons.

Note: DO NOT use a known spent list which includes outputs
which are not known spent. If the list includes any output
that's just strongly thought to be spent, but not provably
so, you risk finding yourself unable to sync past the point
where that output is spent.

I estimate only 200 MB saved on current mainnet though,
unless the new blackballing rule unearths a good amount of
large-amount-set extra spent outs.

tests/unit_tests/testdb.h

@@ -142,5 +142,6 @@ public:
   virtual bool prune_blockchain(uint32_t pruning_seed = 0) { return true; }
   virtual bool update_pruning() { return true; }
   virtual bool check_pruning() { return true; }
+  virtual void prune_outputs(uint64_t amount) {}
 };