Products.CMFActivity.ActivityTool: Improve behaviour on single-node instances.

- Ignore node preference when spawning activities.
  Otherwise, activities which are not spawned with a preferred node will
  get an effective priority penalty compared to same-priority activities
  spawned *with* a node preference, despite both being to execute by the
  same processing node.
- Break activity processing loop when the current processing node is also
  the activity validation node.
  This avoids pathological cases of activity accumulation, for example when
  reindexing an entire site: _recurseCallMethod is spawned in
  processing_node=0, but immediateReindexObject is spawned in
  processing_node=-1 because of serialization_tag dependency, so with such
  loop _recurseCallMethod will be executed over and over, piling indexation
  activities up until _recurseCallMethod does not self-respawn.
  In turn, such activity accumulation lead to an increased overhead, and
  decreased activity processing efficiency.
  This may also allow multi-node instances to more reliably use the
  validation node as a processing node.

The cost for multi-node instances of these changes should be absolutely
minimal (no extra IO necessary, minimal extra code).
A possible drawback on single-node instances is that tic period may become
more important because process_timer will return more often.