Revert "bigfile/zodb: Make ZBlk1 format the default"

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:

    nexedi/slapos@2558aadd

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

/cc @yusei, @klaus, @Tyagov
/reviewed-on nexedi/wendelin.core!5

bigfile/file_zodb.py

@@ -382,7 +382,7 @@ ZBlk_fmt_registry = {
 }
 
 # format for updated blocks
-ZBlk_fmt_write = os.environ.get('WENDELIN_CORE_ZBLK_FMT', 'ZBlk1')
+ZBlk_fmt_write = os.environ.get('WENDELIN_CORE_ZBLK_FMT', 'ZBlk0')
 if ZBlk_fmt_write not in ZBlk_fmt_registry:
     raise RuntimeError('E: Unknown ZBlk format %r' % ZBlk_fmt_write)