Make nSequenceId init value constants

Make it easier to follow what the values come without having to go
over the comments, plus easier to maintain

src/node/blockstorage.cpp

@@ -218,7 +218,7 @@ CBlockIndex* BlockManager::AddToBlockIndex(const CBlockHeader& block, CBlockInde
     // We assign the sequence id to blocks only when the full data is available,
     // to avoid miners withholding blocks but broadcasting headers, to get a
     // competitive advantage.
-    pindexNew->nSequenceId = 1;
+    pindexNew->nSequenceId = SEQ_ID_INIT_FROM_DISK;
 
     pindexNew->phashBlock = &((*mi).first);
     BlockMap::iterator miPrev = m_block_index.find(block.hashPrevBlock);