mvcc: Revisions() method for index to avoid key allocation

[heyitsanthony]

Save another alloc on the one key path.

master:

$ go test -bench=OneKey -run=OneKey -count=4 
BenchmarkStoreRangeOneKey-8      1000000              1883 ns/op             610 B/op         14 allocs/op
BenchmarkStoreRangeOneKey-8      1000000              1807 ns/op             609 B/op         14 allocs/op
BenchmarkStoreRangeOneKey-8      1000000              1861 ns/op             610 B/op         14 allocs/op
BenchmarkStoreRangeOneKey-8      1000000              1850 ns/op             610 B/op         14 allocs/op

branch:

$ go test -bench=OneKey -run=OneKey -count=4
BenchmarkStoreRangeOneKey-8      1000000              1808 ns/op             578 B/op         13 allocs/op
BenchmarkStoreRangeOneKey-8      1000000              1766 ns/op             578 B/op         13 allocs/op
BenchmarkStoreRangeOneKey-8      1000000              1800 ns/op             577 B/op         13 allocs/op
BenchmarkStoreRangeOneKey-8      1000000              1758 ns/op             578 B/op         13 allocs/op

[gyuho]

lgtm. thanks.

[xiang90]

lgtm