Port GetAllLFSPointers to Go

From #1709 (comment 178576099)

Having said that, the RPC is implemented in gitaly-ruby, which is not where we get our best performance. It will not be easy to re-implement this thing because of all the indirection but there is no fundamental reason not to. This should be implementable using just git rev-list and git cat-file.

GetAllLFSPointers isn't performing great on large repositories, and to improve that this RPC should be ported to Golang.