WT_VERBOSE_DEBUG is currently the only "debug" log level. Change the log level enum to include WT_VERBOSE_DEBUG_1 through WT_VERBOSE_DEBUG_5, and convert any uses of WT_VERBOSE_DEBUG to be WT_VERBOSE_DEBUG_1.

Note that this does NOT include making any changes to the tests (which should continue to work fine after making this change) or thinking about whether WT_VERBOSE_DEBUG_1 is an appropriate replacement in all cases - it's purely about getting WT to build with the new definitions.