Retry if releasing lock reduced nchunks. (#646)

@agorrod or @michaelcahill here is one possibility for a fix for the parallel-pop-lsm config for wtperf.  It allows the relevant values of skip_chunks to get recomputed when lsm_tree->nchunks changes, thus avoiding the segv.  However, I know this area only enough to be dangerous.  It does pass the failing wtperf config, make check and run.py.