Reduce memory usage in compute_lfc_stats_multicore

[aksarkar]

This change reduces memory usage by roughly 10%.

Profiling via mprof using the following snippet

devtools::load_all()
data(pbmc_facs)
temp <- de_analysis(pbmc_facs$fit, pbmc_facs$counts, verbose=FALSE, shrink.method='none', control=list(nc=32, nsplit=32))

on an AWS c7i.8xlarge instance yields the following usages.

On 295d732

On g19ec112 :

[pcarbo]

@aksarkar I'm going to hold off on merging your PR for now. I think there is the potential to greatly improve the memory usage, but I have to play around with the implementation, and make sure that it doesn't also slow code the multithreading.

[aksarkar]

The timing of my test case indicates that it slightly improves the wallclock time also. In any case, it does not make appear to make the running time worse.