bigfile/zodb: Apply auto format as default only in WCFS mode
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.
/cc @levin.zimmermann, @klaus, @Tyagov