Reimplement CBlockLocator's chain-related logic in CChain.

This removes a few unused CBlockLocator methods, and moves the
construction and fork-finding logic to CChain (which can do these
more efficiently, as it has a height-indexable chain available).
It also makes CBlockLocator independent from the validation code.

src/db.h

@@ -16,7 +16,7 @@
 #include <db_cxx.h>
 
 class CAddrMan;
-class CBlockLocator;
+struct CBlockLocator;
 class CDiskBlockIndex;
 class CMasterKey;
 class COutPoint;