Currently `TDBStore::offset_in_erase_unit()` and
`TDBStore::check_erase_before_write()` loop through erase units
one-by-one, until the entire range is covered. This is very inefficient
when the erase size is tiny, e.g. one-byte on a non-flash device for
which we use program as erase.

This commit reworks the algorithms, based on the fact that a block
device can erase or program as many units as needed in one go.