improve memory footprint and performance of updates

[erizocosmico]

Depends on #214 and #216, just review last commit

Another dramatic cut in memory usage. This one's even higher:

BenchmarkKallaxUpdateWithRelationships-4   	     200	   6084449 ns/op	   25921 B/op	     570 allocs/op
BenchmarkKallaxUpdate-4                    	     300	   4169555 ns/op	    4201 B/op	     102 allocs/op

BenchmarkKallaxUpdateWithRelationships-4   	     200	   5436011 ns/op	    6643 B/op	     175 allocs/op
BenchmarkKallaxUpdate-4                    	     300	   4010681 ns/op	     656 B/op	      25 allocs/op

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@90dca92). Click here to learn what that means.
The diff coverage is 97.05%.

@@            Coverage Diff            @@
##             master     #217   +/-   ##
=========================================
  Coverage          ?   79.17%           
=========================================
  Files             ?       19           
  Lines             ?     3650           
  Branches          ?        0           
=========================================
  Hits              ?     2890           
  Misses            ?      549           
  Partials          ?      211

Impacted Files	Coverage Δ
store.go	88.1% <97.05%> (ø)

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[ajnavarro]

LGTM