Issue title: munge_sumstats.py stuck and taking too much time
 Issue body: Hi,
Is it normal that it takes more than 30 min to run munge_sumstats.py? The code is stuck for more than 30 min like in the figure below:
```
../munge_sumstats.py  --sumstats 1457_m.txt  --out fornix_big_40 --merge-alleles w_hm3.snplist --N 22138
*********************************************************************
* LD Score Regression (LDSC)
* Version 1.0.1
* (C) 2014-2019 Brendan Bulik-Sullivan and Hilary Finucane
* Broad Institute of MIT and Harvard / MIT Department of Mathematics
* GNU General Public License v3
*********************************************************************
Call:
./munge_sumstats.py \
--out fornix_big_40 \
--merge-alleles w_hm3.snplist \
--N 22138.0 \
--sumstats 1457_m.txt

Interpreting column names as follows:
A1:	Allele 1, interpreted as ref allele for signed sumstat.
P:	p-Value
BETA:	[linear/logistic] regression coefficient (0 --> no effect; above 0 --> A1 is trait/risk increasing)
A2:	Allele 2, interpreted as non-ref allele for signed sumstat.
SNP:	Variant ID (e.g., rs number)

Reading list of SNPs for allele merge from w_hm3.snplist
Read 1217311 SNPs for allele merge.
Reading sumstats from 1457_m.txt into memory 5000000 SNPs at a time.
```
My GWAS file has 16445196 SNPs.
Thanks
 Comments: 
 Comment 0: use `--chunksize 500000`

This is similar to #181
 Comment 1: Works great, thank you!!
{'source': 'GithubIssues', 'repo_name_with_owner': 'bulik/ldsc', 'issue_id': 629453451, 'repo_stars': 431.0}