tdb2: handle non-transaction-page-aligned sizes in recovery.

tdb1 always makes the tdb a multiple of the transaction page size,
tdb2 doesn't.  This means that if a transaction hits the exact end of
the file, we might need to save off a partial page.

So that we don't have to rewrite tdb_recovery_size() too, we simply do
a short read and memset the unused section to 0 (to keep valgrind
happy).