This reverts commit 9ae42085.

When working with big arrays and accessing / changing it not in tiny
bits ZBlk1 is much slower compared to ZBlk0. See details here

    https://www.nexedi.com/blog/NXD-Document.Blog.Wendelin.Core.Release.0.5.Performance.Tests

and in

    13c0c17c (bigfile/zodb: Format #1 which is optimized for small changes)

Until we can rely on database handling both cases automatically,
projects which care about changing arrays in small parts can manually
set WENDELIN_CORE_ZBLK_FMT=ZBlk1 or under ERP5/SlapOS use this setting:

    slapos@2558aadd

And let's have it performant in "big data" case by default.

/cc @yusei, @klaus, @Tyagov
/reviewed-on !5