refactor: remove RecursiveMutex cs_nBlockSequenceId

The RecursiveMutex cs_nBlockSequenceId is only used at one place in
CChainState::ReceivedBlockTransactions() to atomically read-and-increment the
nBlockSequenceId member. At this point, the cs_main lock is set, hence we can
use a plain int for the member and mark it as guarded by cs_main.

src/validation.h

@@ -558,9 +558,8 @@ protected:
      * Every received block is assigned a unique and increasing identifier, so we
      * know which one to give priority in case of a fork.
      */
-    RecursiveMutex cs_nBlockSequenceId;
     /** Blocks loaded from disk are assigned id 0, so start the counter at 1. */
-    int32_t nBlockSequenceId = 1;
+    int32_t nBlockSequenceId GUARDED_BY(::cs_main) = 1;
     /** Decreasing counter (used by subsequent preciousblock calls). */
     int32_t nBlockReverseSequenceId = -1;
     /** chainwork for the last block that preciousblock has been applied to. */
@@ -749,7 +748,7 @@ public:
 
     void PruneBlockIndexCandidates();
 
-    void UnloadBlockIndex();
+    void UnloadBlockIndex() EXCLUSIVE_LOCKS_REQUIRED(::cs_main);
 
     /** Check whether we are doing an initial block download (synchronizing from disk or network) */
     bool IsInitialBlockDownload() const;