Use Equihash for Proof-of-Work

The main and test networks are configured to use parameters that are currently
low-memory but usable with the basic solver; they will be increased once the
solver is optimised. The regtest network is configured to have extremely low
memory usage for speed.

Note that Bitcoin's double-hasher is used for the difficulty check. This does
not match the paper, but is simpler than changing the block header
serialization. Single hashing is kept for the EquiHash solver because there is
no requirement on execution time there, only on memory usage.

src/txdb.cpp

@@ -308,6 +308,7 @@ bool CBlockTreeDB::LoadBlockIndexGuts()
                 pindexNew->nTime          = diskindex.nTime;
                 pindexNew->nBits          = diskindex.nBits;
                 pindexNew->nNonce         = diskindex.nNonce;
+                pindexNew->nSolution      = diskindex.nSolution;
                 pindexNew->nStatus        = diskindex.nStatus;
                 pindexNew->nTx            = diskindex.nTx;