If current chunk size is lesser than maximum, it means latest check for the
whole chunk found a problem. If current smaller check succeeded, move to
the other half and divide size by two.
Also, factorise replication "check...Range" handlers.

git-svn-id: https://svn.erp5.org/repos/neo/trunk@2405 71dcc9de-d417-0410-9af5-da40c76e7ee4