Separate PluginLoggerImpl from LoggerImpl

Since all methods in LoggerImpl were checking
`if (runAsPlugin < 0) {...}` separating out the logic
into a separate class is straight-forward. The reason
for doing this, however, is that in the case that the
IJ plugin is used with the `loci_tools.jar` jar, which
contains log4j, then the client hangs at

    "Loading Logger Service"

--depends-on openmicroscopy/bioformats#889
--depends-on openmicroscopy/openmicroscopy#2074