mvcc: don't allocate keys when computing Revisions

[heyitsanthony]

~6% improvement

Before:

$ go test -bench=RangeKey100 -run=RangeKey100 -v  -count=4
BenchmarkStoreRangeKey100-8        10000            118881 ns/op           46240 B/op        723 allocs/op
BenchmarkStoreRangeKey100-8        10000            117498 ns/op           46241 B/op        724 allocs/op
BenchmarkStoreRangeKey100-8        10000            119148 ns/op           46241 B/op        724 allocs/op
BenchmarkStoreRangeKey100-8        10000            116682 ns/op           46241 B/op        724 allocs/op

After:

BenchmarkStoreRangeKey100-8        10000            111767 ns/op           40113 B/op        716 allocs/op
BenchmarkStoreRangeKey100-8        10000            110846 ns/op           40112 B/op        715 allocs/op
BenchmarkStoreRangeKey100-8        10000            111666 ns/op           40112 B/op        715 allocs/op
BenchmarkStoreRangeKey100-8        10000            111672 ns/op           40113 B/op        716 allocs/op

[gyuho]

lgtm. thanks!