Require CBlockIndex::IsValid() to hold cs_main

Summary:
This is a partial backport of [[bitcoin/bitcoin#22932 | core#22932]]
bitcoin/bitcoin@ca47b00

Depends on D13035

Test Plan:
With clang and DEBUG:

`ninja all check-all`

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13036

src/blockindex.h

@@ -206,7 +206,9 @@ class CBlockIndex {
 
     //! Check whether this block index entry is valid up to the passed validity
     //! level.
-    bool IsValid(enum BlockValidity nUpTo = BlockValidity::TRANSACTIONS) const {
+    bool IsValid(enum BlockValidity nUpTo = BlockValidity::TRANSACTIONS) const
+        EXCLUSIVE_LOCKS_REQUIRED(::cs_main) {
+        AssertLockHeld(::cs_main);
         return nStatus.isValid(nUpTo);
     }