common: make LogChannel mockable

common: make LogChannel mockable

Adding LoggerSinkSet, an abstract API that enables unit-testing
of OSD components that emit cluster-log messages, and trapping the
logs.

First usage example is in the Scrubber backend unit-tests.
The log "implementation" there (note the error counter that
can be used to determine tests success):

 class MockLog : public LoggerSinkSet {
 public:
  void warn(std::stringstream& s) override
  {
    std::cout << "\n<<warn>> " << s.str() << std::endl;
  }
  void error(std::stringstream& s) override
  {
    err_count++;
    std::cout << "\n<<error>> " << s.str() << std::endl;
  }
  ...

Signed-off-by: Ronen Friedman <rfriedma@redhat.com>