Change LogAcceptCategory to use uint32_t rather than sets of strings.

This changes the logging categories to boolean flags instead of strings.

This simplifies the acceptance testing by avoiding accessing a scoped
 static thread local pointer to a thread local set of strings.  It
 eliminates the only use of boost::thread_specific_ptr outside of
 lockorder debugging.

This change allows log entries to be directed to multiple categories
 and makes it easy to change the logging flags at runtime (e.g. via
 an RPC, though that isn't done by this commit.)

It also eliminates the fDebug global.

Configuration of unknown logging categories now produces a warning.

src/dbwrapper.cpp

@@ -21,8 +21,9 @@ public:
     // This code is adapted from posix_logger.h, which is why it is using vsprintf.
     // Please do not do this in normal code
     virtual void Logv(const char * format, va_list ap) override {
-            if (!LogAcceptCategory("leveldb"))
+            if (!LogAcceptCategory(BCLog::LEVELDB)) {
                 return;
+            }
             char buffer[500];
             for (int iter = 0; iter < 2; iter++) {
                 char* base;