Simplify idle_loop

Tested for no regression in SMP.

STC with 3 threads
LLR: 2.96 (-2.94,2.94) [-3.50,0.50]
Total: 52769 W: 7929 L: 7894 D: 36946

30 secs per game with 3 threads
LLR: 2.96 (-2.94,2.94) [-3.50,0.50]
Total: 57619 W: 7848 L: 7816 D: 41955

And STC with 7 threads
LLR: 2.96 (-2.94,2.94) [-6.00,0.00]
Total: 19901 W: 2789 L: 2772 D: 14340

No functional change.
Small tweak to idle_loop()

In case of a succesful late join we set again
'searching' flag, so we can restart search
immediately without an useless lock/unlock
cycle.

No functional change.