Limit the impact of reorganisations on the database

Sometimes a new block arrives in a new chain that was already the
best valid one, but wasn't marked that way. This happens for example
when network rules change to recover after a fork.

In this case, it is not necessary to do the entire reorganisation
inside a single db commit. These can become huge, and exceed the
objects/lockers limits in bdb. This patch limits the blocks the
actual reorganisation is applied to, and adds the next blocks
afterwards in separate db transactions.

src/main.h

@@ -1030,6 +1030,9 @@ public:
     bool AddToBlockIndex(unsigned int nFile, unsigned int nBlockPos);
     bool CheckBlock() const;
     bool AcceptBlock();
+
+private:
+    bool SetBestChainInner(CTxDB& txdb, CBlockIndex *pindexNew);
 };