Make NUM_XLOGINSERT_LOCKS configurable

Dear all,

I recently used benchmarksql to evaluate the performance of postgresql. I achieved nearly 20% improvement 
with NUM_XLOGINSERT_LOCKS changed from 8 to 16 under some cases of high concurrency. I wonder whether 
it is feasible to make NUM_XLOGINSERT_LOCKS a configuration parameter, so that users can get easier to optimize 
their postgresql performance through this setting.

Thanks,
Qingsong