Refactor logging to allow for complete control when used as a library

The logging system in manticore.utils.log currently makes global changes like setting the the logging logger class. At best, this hinders third party programs that use Manticore as a library to modify how Manticore logs things. At worst, this can actually break third party programs' loggers.

As a continuation of #688, refactor the logging framework to make it extensible when Manticore is used as a library.