Fixes the following scenario (committing an object):
2 storage nodes: S1 and S2
1 replicas
- store in S1 (1)
- store in S2 (2)
- S1 answers (1) with a conflict
- client resolves the conflict, and sends to S1 (3) and S2 (4)
- S1 answers (3) with ok
- S2 answers (2) with a conflict

Old code raises because object store counter was not 0 on an object for
which a conflict was reported.
New code ignores answers notifying of a conflict if given serial is the
already-resolved conflict.
Split & extend existing tests.

git-svn-id: https://svn.erp5.org/repos/neo/trunk@2015 71dcc9de-d417-0410-9af5-da40c76e7ee4