Minor re-working of 9dd055a.

Panic in the block layer if we see page-level corruption failures even
if it's not in DIAGNOSTIC mode, there's no reason to keep running.

Don't panic in the btree layer if we're doing a verify and there's a
decompression failure, verify should continue just like salvage.

Rename the SALVAGE_QUIET_ERR flag to SALVAGE_CORRUPT_OK, for clarity.
Flag row-store leaf pages with no empty values, avoiding a second pass
over the page when it's read into memory.

Add verify code to check the number of keys vs. physical entries on
row-store internal pages and row-store leaf pages where the page has
no empty values.

Move the code to confirm the first byte after the last cell on a page
matches the length of the page into verify and do the check for all page
types that have cells (it was previously in the read code and we only
checked row-store leaf pages).
Merge pull request #834 from wiredtiger/empty-values

Avoid second pass over row-store leaf pages when they're read.
Merge pull request #833 from wiredtiger/salvage-corrupt

Minor re-working of checksum error handling.