A lot of times data in blocks come shorter than block size and the rest
of the memory page is zeros (because it was pre-filled zeros by OS when
page was allocated).

Do a simple heuristic and trim those trailing zeros and not store them
into DB.

With this change size of DB created by running bigfile and bigarray
tests changes as following:

            bigfile     bigarray

    old     145M         35M
    new      24K          6M

Trimming trailing zeros is currently done with str.rstrip('\0') which
creates a copy. When/if needed this could be optimized to work in-place.