Create -loglevel configuration option

- add a -loglevel=<level>|<category:level> config option to allow users
  to set a global -loglevel and category-specific log levels. LogPrintLevel
  messages with a higher severity level than -loglevel will not be printed
  in the debug log.

- for now, this config option is debug-only during the migration to
  severity-based logging

- update unit and functional tests

Co-authored-by: "Jon Atack <jon@atack.com>"

src/test/util/setup_common.cpp

@@ -108,6 +108,7 @@ BasicTestingSetup::BasicTestingSetup(const std::string& chainName, const std::ve
             "-logsourcelocations",
             "-logtimemicros",
             "-logthreadnames",
+            "-loglevel=debug",
             "-debug",
             "-debugexclude=libevent",
             "-debugexclude=leveldb",