bigfile/zodb: Make ZBlk1 format the default

Our current workloads are mostly a lot of small data changes and this is what
ZBlk1 was created for. Yes it has larger overhead for accessing data, but we
already painted the way how to handle this in 13c0c17c (bigfile/zodb: Format #1
which is optimized for small changes) -> move data deduplication/management to
server side.

So be it ZBlk1 the default for now.

/cc @Tyagov, @klaus

bigfile/file_zodb.py

@@ -347,7 +347,7 @@ ZBlk_fmt_registry = {
 }
 
 # format for updated blocks
-ZBlk_fmt_write = os.environ.get('WENDELIN_CORE_ZBLK_FMT', 'ZBlk0')
+ZBlk_fmt_write = os.environ.get('WENDELIN_CORE_ZBLK_FMT', 'ZBlk1')
 if ZBlk_fmt_write not in ZBlk_fmt_registry:
     raise RuntimeError('E: Unknown ZBlk format %r' % ZBlk_fmt_write)