MINOR: listener: resync with the thread index before heavy calculations

MINOR: listener: resync with the thread index before heavy calculations

During heavy accept competition, the CAS will occasionally fail and
we'll have to go through all the calculation again. While the first
two loops look heavy, they're almost never taken so they're quite
cheap. However the rest of the operation is heavy because we have to
consult connection counts and queue indexes for other threads, so
better double-check if the index is still valid before continuing.
Tests show that it's more efficient do retry half-way like this.