Add ability to turn off log categories by default (PLAT-564)

Adds an extra boolean parameter to the log category that allows it to be turned off by default, to support the 'recompile with verbose logging' use-case.  This could be combined at a later stage with the ability to modify logging via environment variables or a SHM segment and a signal.
Interestingly enough, on GCC 4.6 with the following code:

```c++
    // (e is a log category)
    BOOST_CHECK(!e);
    BOOST_CHECK_EQUAL((int)e, 0);  // <-- A
    int i = e;
    BOOST_CHECK_EQUAL(i, 0);        // <-- B
```

With the "explicit bool" we get a compile error on just B

./soa/service/testing/logs_test.cc:51:13: error: cannot convert ‘Datacratic::Logging::Category’ to ‘int’ in initialization

With the PMF conversion, we get a compilation error on A as well

./soa/service/testing/logs_test.cc:50:607: error: invalid cast from type ‘Datacratic::Logging::Category’ to type ‘int’
./soa/service/testing/logs_test.cc:51:13: error: cannot convert ‘Datacratic::Logging::Category’ to ‘int’ in initialization

So they're (surprisingly) not equivalent.