Reduce memory usage for kmer mapper normalization #111

Job	Run time
🚧 Build SPAdes	3m 55s
Full run M.ruber JGI lane 9	5m 44s
Multicell B.cereus TruSeq 100x	4m 34s
IonTorrent E.coli K12	28m 8s
Mate-pairs B.faecium	2m 14s
Contigs E.coli JGI lane 7	3m 41s
Nextera L.monocytogenes	1m 4s
plasmidSPAdes C.callunae	10m 42s
Single-cell P.heparinus JGI lane 7	5m 35s
metaviralSPAdes ERR525599	15m 42s
metaSPAdes JGI 150	18m 45s
PacBio M.ruber	4m 23s
Saves E.coli 100K	45s
Saves E.coli UCSD lane 1	9m 21s
Single reads E.coli Deeptrench	4m 53s
	1h 59m 26s

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