New database check routine

-checklevel gets a new meaning:
0: verify blocks can be read from disk (like before)
1: verify (contextless) block validity (like before)
2: verify undo files can be read and have good checksums
3: verify coin database is consistent with the last few blocks
   (close to level 6 before)
4: verify all validity rules of the last few blocks

Level 3 is the new default, as it's reasonably fast. As level 3 and
4 are implemented using an in-memory rollback of the database, they
are limited to as many blocks as possible without exceeding the
limits set by -dbcache. The default of -dbcache=25 allows for some
150-200 blocks to be rolled back.

In case an error is found, the application quits with a message
instructing the user to restart with -reindex. Better instructions,
and automatic recovery (when possible) or automatic reindexing are
left as future work.

src/main.h

@@ -126,6 +126,8 @@ FILE* OpenUndoFile(const CDiskBlockPos &pos, bool fReadOnly = false);
 bool LoadExternalBlockFile(FILE* fileIn, CDiskBlockPos *dbp = NULL);
 /** Load the block tree and coins database from disk */
 bool LoadBlockIndex();
+/** Verify consistency of the block and coin databases */
+bool VerifyDB();
 /** Print the loaded block tree */
 void PrintBlockTree();
 /** Find a block by height in the currently-connected chain */