Divide conflict hot-spot to improve write performance: now, conflict risk
will not be proportional to the number of zope processes, but only to the
number of threads within considered process (...because there is no
stable thread name, if there is one day and conflict splitting is needed,
it will be easy to implement: just concatenate that name to node name in
FragmentedLength._map).
Automatically migrate to FragmentedCount on containers larger than 1k
objects (this threshold may evolve).