postgres: some perf work

[hdonnay]

This is a stab at some memory optimizations that jumped out. This now uses less than half the memory for calculating a hash:

% go test -run none -bench Hash\|MD5        
goos: linux
goarch: amd64
pkg: github.com/quay/claircore/datastore/postgres
cpu: 11th Gen Intel(R) Core(TM) i7-11850H @ 2.50GHz
BenchmarkMD5Vuln/Baseline-16     	  886275	      1622 ns/op	     648 B/op	      10 allocs/op
BenchmarkMD5Vuln/Current-16      	 1000000	      1195 ns/op	     232 B/op	       7 allocs/op
BenchmarkVulnerabilityHash/MD5-16         	 1000000	      1259 ns/op	     232 B/op	       7 allocs/op
BenchmarkVulnerabilityHash/XXH64-16       	 1379953	       863.0 ns/op	     224 B/op	       7 allocs/op
PASS
ok  	github.com/quay/claircore/datastore/postgres	5.093s

[crozzy]

SGTM

[hdonnay]

/fast-forward