Adds getToppersAsync() for async computation

A method which is used for asynchronously computing the top-Accounts.
It will copy the origin Accounts, then asynchronously execute the sort
task and then return the Future which will have the sorted copy.
This method is useful if you want a non-blocking way of getting the
top-accounts, provided the get() method is also in a separate thread
called, which is unhooked from the main-thread.

Signed-off-by: XemsDoom <moser.luca@gmail.com>