Add missing presto-main dependency to presto-server

This was missed in the refactoring that moved the server code to presto-main.
As a result, the server fails to start when run via the launcher
Replace Unsafe with new Slice ByteArrays utility
Remove obsolete TODO
Set 1 hour time bound on model training
Remove heuristic from learn aggregation
Add unit test for learn aggregation
Add function to train libsvm models

Add special learning functions that allow you to configure the
hyperparameters of the model