wendelin.core: v↑ 2.0.alpha3 -> 2.0.alpha4

New 'wendelin.core' version contains several bugfixes that were
discovered in field usage. Furthermore it introduces the new 'ZBlk_fmt'
option 'auto' that heuristically sets ZBlkfmt to gain a good ratio between
read time and database size, improves python3 compatibility and synchronizes
NEO zurl scheme between NEO/py and NEO/go.

See nexedi/wendelin.core@63153845...db6fea3d
for complete list of changes.

component/wendelin.core/buildout.cfg

@@ -50,7 +50,7 @@ CGO_LDFLAGS += -Wl,-rpath=${zlib:location}/lib
 recipe  = slapos.recipe.build:gitclone
 repository = https://lab.nexedi.com/nexedi/wendelin.core.git
 branch   = master
-revision = wendelin.core-2.0.alpha3-22-g07087ec8
+revision = wendelin.core-2.0.alpha4-0-gdb6fea3
 # dir is pretty name as top-level recipe
 location = ${buildout:parts-directory}/wendelin.core
 git-executable = ${git:location}/bin/git