refactor: validation: mark SnapshotBase as const

It does not modify any logical state. This change is required
for future const-correctness commits.

src/validation.cpp

@@ -1970,7 +1970,7 @@ Chainstate::Chainstate(
       m_chainman(chainman),
       m_from_snapshot_blockhash(from_snapshot_blockhash) {}
 
-const CBlockIndex* Chainstate::SnapshotBase()
+const CBlockIndex* Chainstate::SnapshotBase() const
 {
     if (!m_from_snapshot_blockhash) return nullptr;
     if (!m_cached_snapshot_base) m_cached_snapshot_base = Assert(m_chainman.m_blockman.LookupBlockIndex(*m_from_snapshot_blockhash));

src/validation.h

@@ -532,7 +532,7 @@ protected:
     bool m_disabled GUARDED_BY(::cs_main) {false};
 
     //! Cached result of LookupBlockIndex(*m_from_snapshot_blockhash)
-    const CBlockIndex* m_cached_snapshot_base GUARDED_BY(::cs_main) {nullptr};
+    mutable const CBlockIndex* m_cached_snapshot_base GUARDED_BY(::cs_main){nullptr};
 
 public:
     //! Reference to a BlockManager instance which itself is shared across all
@@ -596,7 +596,7 @@ public:
      *
      * nullptr if this chainstate was not created from a snapshot.
      */
-    const CBlockIndex* SnapshotBase() EXCLUSIVE_LOCKS_REQUIRED(::cs_main);
+    const CBlockIndex* SnapshotBase() const EXCLUSIVE_LOCKS_REQUIRED(::cs_main);
 
     /**
      * The set of all CBlockIndex entries that have as much work as our current