MEDIUM: task: perform a single tree lookup per run queue batch

MEDIUM: task: perform a single tree lookup per run queue batch

The run queue is designed to perform a single tree lookup and to
use multiple passes to eb32sc_next(). The scheduler rework took a
conservative approach first but this is not needed anymore and it
increases the processing cost of process_runnable_tasks() and even
the time during which the RQ lock is held if the global queue is
heavily loaded. Let's simply move the initial lookup to the entry
of the loop like the previous scheduler used to do. This has reduced
by a factor of 5.5 the number of calls to eb32sc_lookup_get() there.