Set the same best tip on restart if two candidates have the same work

Before this, if we had two (or more) same work tip candidates and restarted our node,
it could be the case that the block set as tip after bootstrap didn't match the one
before stopping. That's because the work and `nSequenceId` of both block will be the same
(the latter is only kept in memory), so the active chain after restart would have depended
on what tip candidate was loaded first.

This makes sure that we are consistent over reboots.

src/node/blockstorage.cpp

@@ -166,7 +166,8 @@ bool CBlockIndexWorkComparator::operator()(const CBlockIndex* pa, const CBlockIn
     if (pa->nSequenceId > pb->nSequenceId) return true;
 
     // Use pointer address as tie breaker (should only happen with blocks
-    // loaded from disk, as those all have id 0).
+    // loaded from disk, as those share the same id: 0 for blocks on the
+    // best chain, 1 for all others).
     if (pa < pb) return false;
     if (pa > pb) return true;
 
@@ -217,7 +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 = 0;
+    pindexNew->nSequenceId = 1;
 
     pindexNew->phashBlock = &((*mi).first);
     BlockMap::iterator miPrev = m_block_index.find(block.hashPrevBlock);