logging: add LogError, LogWarning, LogInfo, LogDebug, LogTrace

These provide simple and clear ways to write the most common logging
operations:

    LogInfo("msg");
    LogDebug(BCLog::LogFlags::NET, "msg");

    LogError("msg");
    LogWarning("msg");
    LogTrace(BCLog::LogFlags::NET, "msg");

For cases where the level cannot be hardcoded, LogPrintLevel(category,
level, ...) remains available.

test/lint/lint-format-strings.py

@@ -20,6 +20,11 @@ FUNCTION_NAMES_AND_NUMBER_OF_LEADING_ARGUMENTS = [
     'fprintf,1',
     'tfm::format,1',  # Assuming tfm::::format(std::ostream&, ...
     'LogConnectFailure,1',
+    'LogError,0',
+    'LogWarning,0',
+    'LogInfo,0',
+    'LogDebug,1',
+    'LogTrace,1',
     'LogPrint,1',
     'LogPrintf,0',
     'LogPrintfCategory,1',