[cdc_rsync] Improve throughput for local copies (#74)

On Windows, fclose() seems to be very expensive for large files, where
closing a 1 GB file takes up to 5 seconds. This CL calls fclose() in
background threads. This tremendously improves local syncs, e.g.
copying a 4.5 GB, 300 files data set takes only 7 seconds instead of
30 seconds.

Also increases the buffer size for copying from 16K to 128K (better
throughput for local copies), and adds a timestamp to debug and
verbose console logs (useful when comparing client and server logs).

common/log.h

@@ -22,6 +22,7 @@
 #include "absl/strings/str_format.h"
 #include "absl/synchronization/mutex.h"
 #include "common/clock.h"
+#include "common/stopwatch.h"
 
 namespace cdc_ft {
 
@@ -120,6 +121,7 @@ class ConsoleLog : public Log {
       ABSL_LOCKS_EXCLUDED(mutex_);
 
  private:
+  Stopwatch stopwatch_;
   absl::Mutex mutex_;
 };