Implement "getchaintips" RPC command to monitor blockchain forks.

Port over https://github.com/chronokings/huntercoin/pull/19 from
Huntercoin:  This implements a new RPC command "getchaintips" that can be
used to find all currently active chain heads.  This is similar to the
-printblocktree startup option, but it can be used without restarting
just via the RPC interface on a running daemon.

src/main.h

@@ -1068,7 +1068,7 @@ public:
     CBlockIndex *FindFork(const CBlockLocator &locator) const;
 
     /** Find the last common block between this chain and a block index entry. */
-    CBlockIndex *FindFork(CBlockIndex *pindex) const;
+    const CBlockIndex *FindFork(const CBlockIndex *pindex) const;
 };
 
 /** The currently-connected chain of blocks. */