Add -checklevel and improve -checkblocks

-checkblocks now takes a numeric argument: the number of blocks that must
be verified at the end of the chain. Default is 2500, and 0 means all
blocks.

-checklevel specifies how thorough the verification must be:
0: only check whether the block exists on disk
1: verify block validity (default)
2: verify transaction index validity
3: check transaction hashes
4: check whether spent txouts were spent within the main chain
5: check whether all prevouts are marked spent
6: check whether spent txouts were spent by a valid transaction that consumes them