Provide per-package loggers.

I also made them package private where possible. It's too easy for code that
depends on samskivert to accidentally import com.samskivert.Log rather than
define its own logger.

Unfortunately Java doesn't have "this package and subpackages" protection, so
some of the package loggers have to be public to allow sub-packages to access
them. I didn't really want to have to define loggers for half a dozen
subpackages as well.