Add -reindex, to perform in-place reindexing of block chain files

Flushes the blktree/ and coins/ databases, and reindexes the
block chain files, as if their contents was loaded via -loadblock.

Based on earlier work by Jeff Garzik.

src/leveldb.h

@@ -69,7 +69,7 @@ private:
     leveldb::DB *pdb;
 
 public:
-    CLevelDB(const boost::filesystem::path &path, size_t nCacheSize, bool fMemory = false);
+    CLevelDB(const boost::filesystem::path &path, size_t nCacheSize, bool fMemory = false, bool fWipe = false);
     ~CLevelDB();
 
     template<typename K, typename V> bool Read(const K& key, V& value) {