[Reviewer: Andy] Rework the fake logger used in UTs

Mike, Andy,

Can you give this and https://github.com/Metaswitch/cpp-common/pull/120 a quick look and see if you agree with the direction I'm taking? This isn't quite finished and needs polishing and porting to the other cpp-common users, but I wanted to make sure I'm not doing anything fundamentally wrong.

I've broken FakeLogger up into:
* a PrintingTestLogger, with a static instance, PrintingTestLogger::DEFAULT. This has methods to change the logging level and noisiness, but by default it honours the NOISY environment variable
* a CapturingTestLogger, designed to be created when you have a test that needs to check a log file, just within the scope of that test. On construction, it sets the log level to maximum to avoid false negatives. On destruction, it hands control back to PrintingTestLogger::DEFAULT, which resets logging to the level of the NOISY environment variable. (I only realised it should do this in the destructor a bit before my battery died, so there are still some explicit calls to PrintingTestLogger::DEFAULT.take_over() dotted around).

The rules for creating them are much clearer:
* you never create a PrintingTestLogger because none of them can behave differently from the default
* you create a CapturingTestLogger on the stack inside any test functions that need it

Consequently I've:
* removed the per-test class FakeLogger member
* removed the static pointer to FakeLogger we had somewhere
* removed the calls to Log::setLoggingLevel(99) scattered through random constructors.
Yep, scrap my second comment, I misread.  The first still stands though I think, can we at least assert that either `_logger == NULL` or `_logger == _default_logger`?