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 coin database is consistent with the last few blocks
   (close to level 6 before)
3: verify all validity rules of the last few blocks

Level 2 is the new default, as it's reasonably fast. As level 2 and
3 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
100-150 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.