Replace arrays with slices

[Allda]

This solution should be easier to read and maintain because there is no need of using indexes.

NOTE: There are few more places which use indexes. If this PR will be approved I can refactor rest of codebase.

[jzelinskie]

I suspect that based on the comment above the code:

// TODO(Sida): A better interface for bulk insertion is needed.

This is done for performance reasons. append is going to allocate whenever adding the new value will exceed the existing cap, then it'll double the size of the slice.

The existing code allocates only once.

[jzelinskie]

I think we can get the best of both worlds by keeping the usage of make, followed by using append.

keys := make([]interface{}, len(features)*3)
for _, f := range features {
  keys = append(keys, f.Name, f.Version, f.VersionFormat)
  ...
}

[Allda]

This doesn't seem to work because make([]interface{}, len(features)*3) creates a slice with default values and append() adds items at the end.

[jzelinskie]

Ah, you probably have to do make([]interface{}, 0, len(features)*3) so that you set the capacity and not the length.

[Allda]

Now it works fine after setting just capacity.