This semantically reverts 99f262dd (bigfile/zodb: Make auto format the
default) for wendelin.core-1 mode because in non-WCFS mode there are
known problems with data corruption on BTree topology changes(*) and
auto mode actually does change those topologies with first setting
ZBigFile[blk] -> ZBlk1 and then updating the same block to point to
ZBlk0 object.

Avoid pressuring those problems and use auto as default only in WCFS
mode that should handle invalidations with all those BTree topology
changes well.

The patch is based on suggestion by Levin Zimmermann: !20 (comment 212405)

We have to move _default_use_wcfs because now it is invoked at module
import time and needs to be already defined at the time of the call.

(*) see 8c32c9f6 for details.

/reviewed-by @levin.zimmermann
/reviewed-on !29