Extracted Log interface from Logger class to ease testing

* Should be more used within iPOJO, not only tests
* Backward compatible (levels definition in API and impl)
Introduced a QueueService for asynchronous job scheduling

* 2 implementations: synchronous and asynchronous
* QueueService has a reporting/statistic API that would require an Arch new command
* Error reporting may be done in EventAdmin (TBD)
* QueueService usage in extender should be configurable (enforced or preference QS)

NOTE: This is clearly 'on the edge', itests are actually failing