Simplify and reduce code duplication in Transaction constructors

- Leverage delegating constructor to avoid code duplication between the
  two available Transaction constructors.
  - The constructor without 'id' argument delegates to the one that
    receives it by providing `nullptr` as a value, which is used to
    flag that an id needs to be generated.
- Simplified constructor by removing member initialization where the
  default constructor will be invoked.

src/audit_log/writer/parallel.cc

@@ -49,7 +49,7 @@ Parallel::~Parallel() {
 }
 
 
-inline std::string Parallel::logFilePath(time_t *t,
+inline std::string Parallel::logFilePath(const time_t *t,
     int part) {
     std::string name;
 

src/audit_log/writer/parallel.h

@@ -65,7 +65,7 @@ class Parallel : public Writer {
      YearMonthDayAndTimeFileName = 8,
     };
 
-    static inline std::string logFilePath(time_t *t, int part);
+    static inline std::string logFilePath(const time_t *t, int part);
 };
 
 }  // namespace writer