doco
ExecutorPreference::findAvailableSlot returns int instead of boolean.

Also add getTaskCount, which returns number of tasks in the executor.  The idea is findAvailableSlot will soon return the number of tasks that weren't locally scheduled, so that we can get a task-granular locality report.

Right now only two values are returned: getTaskCount() for false and 0 for true.  In a minute we'll refine the zero.
Actually, return the happy task count.
Promote hostCounts to a member to return real happy task counts.
Log an aggregate report of locally scheduled and indifferent tasks.