[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/BUILD

@@ -158,6 +158,7 @@ cc_library(
     deps = [
         ":clock",
         ":platform",
+        ":stopwatch",
         "@com_google_absl//absl/strings:str_format",
         "@com_google_absl//absl/synchronization",
     ],