Saves a tiny bit of cpu and some IO. Sadly doesn't prevent all IO on
duplicate writes as the transactions are still created and committed.

$ go test -bench=. -benchmem
goos: linux
goarch: amd64
pkg: github.com/hashicorp/nomad/helper/boltdd
BenchmarkWriteDeduplication_On-4             500           4059591 ns/op           23736 B/op         56 allocs/op
BenchmarkWriteDeduplication_Off-4            300           4115319 ns/op           25942 B/op         55 allocs/op