logging: Dedicated log flush timer

Log flushing is currently triggered based on the threading heartbeat timer
of WriterBackends and the hard-coded WRITE_BUFFER_SIZE 1000.

This change introduces a separate timer that is managed by the logger
manager instead of piggy-backing on the heartbeat timer, as well as a
const &redef for the buffer size.

This allows to modify the log flush frequency and batch size independently
of the threading heartbeat interval. Later, this will allow to re-use the
buffering and flushing logic of writer frontends for non-Broker cluster
backends, too.

One change here is that even frontends that do not have a backend will
be flushed regularly. This is wanted for non-Broker backends and should be
very cheap. Possibly, Broker can piggy back on this timer down the road, too,
rather than using its own script-level timer (see Broker::log_flush()).

src/logging/WriterFrontend.h

@@ -261,9 +261,7 @@ protected:
     int num_fields;                        // The number of log fields.
     const threading::Field* const* fields; // The log fields.
 
-    // Buffer for bulk writes.
-    static const int WRITER_BUFFER_SIZE = 1000;
-    detail::WriteBuffer write_buffer; // Buffer of size WRITER_BUFFER_SIZE.
+    detail::WriteBuffer write_buffer; // Buffer for bulk writes.
 };
 
 } // namespace zeek::logging