ThreadService: fix typos in javadoc
ServiceHelper: print timings when in debug mode
Fix Eclipse warnings
LogService: allow log level to be set contextually

You can specific the log level for a specific class, or any package
(which will automatically include subpackages too).

For example, you could debug service loading with:

  System.setProperty(
    "scijava.log.level:org.scijava.service.ServiceHelper", "debug");

There may be performance implications with this implementation:
as soon as *any* custom log levels are set, *every* getLevel()
call triggers some substantial string manipulation logic.
But as usual, we can worry about optimizing later as needed.