When there are many simultaneously-pending activities attached to any
processing node family, the
  node>=0
subquery becomes dominant (taking hundreds of time longer than the other
subqueries). As a consequence, this starves processing nodes of activities
and increases the CPU needs of the mariadb process hosting the activity
tables.
So, move this subquery out of the regular codepath, and only run it if no
other subquery found any activity:
- there is no activity preferentially targeting the current node
- there is no activity bound to any of the current node's families
- there is no activity without any node preference at all
Also, simplify the content of that subquery: the effective priority can
only be 3 * priority + 1 when this query is run, and node=0 rows can be
excluded (they should not exist in the current database view).
Also, factorise the logic producing "node=processing_node" and
"node IN node_set" subqueries, for simplicity. In turn, this makes all
family-dependent subqueries use a simple equality test, ensuring a stable
query plan independently from the number of families the current node is
member of.
Also, use "UNION ALL" always, as now:
- all subqueries have stritly distinct result sets
- as per mariadb documentation, "UNION [DISTINCT] applies to all UNIONs
  on the left", so the original comment about where ALL is used was
  incorrect in assuming it was improving the effective query performance
Also, line-split SQL queries as visible in the python source to be more
readable, without effect on the produced SQL.
Also, line-split a few non-trivial python expression to make their
internal structure immediately apparent.

Another effect of this change this change is to reduce activity theft
(activities to be preferentially executed by one node being executed by
another), potentially improving object cache hit-rate and hence decreasing
I/O pressure on the ZODB.