blockstorage: segment normal/assumedvalid blockfiles

When using an assumedvalid (snapshot) chainstate along with a background
chainstate, we are syncing two very different regions of the chain
simultaneously. If we use the same blockfile space for both of these
syncs, wildly different height blocks will be stored alongside one
another, making pruning ineffective.

This change implements a separate blockfile cursor for the assumedvalid
chainstate when one is in use.

src/validation.cpp

@@ -2601,7 +2601,7 @@ bool Chainstate::FlushStateToDisk(
                 // First make sure all block and undo data is flushed to disk.
                 // TODO: Handle return error, or add detailed comment why it is
                 // safe to not return an error upon failure.
-                if (!m_blockman.FlushBlockFile()) {
+                if (!m_blockman.FlushChainstateBlockFile(m_chain.Height())) {
                     LogPrintLevel(BCLog::VALIDATION, BCLog::Level::Warning, "%s: Failed to flush block file.\n", __func__);
                 }
             }
@@ -5269,6 +5269,7 @@ bool ChainstateManager::ActivateSnapshot(
         assert(chaintip_loaded);
 
         m_active_chainstate = m_snapshot_chainstate.get();
+        m_blockman.m_snapshot_height = this->GetSnapshotBaseHeight();
 
         LogPrintf("[snapshot] successfully activated snapshot %s\n", base_blockhash.ToString());
         LogPrintf("[snapshot] (%.2f MB)\n",