Docker pull/push with max concurrency limits.

This fix tries to address issues raised in #20936 and #22443
where `docker pull` or `docker push` fails because of the
concurrent connection failing.
Currently, the number of maximum concurrent connections is
controlled by `maxDownloadConcurrency` and `maxUploadConcurrency`
which are hardcoded to 3 and 5 respectively. Therefore, in
situations where network connections don't support multiple
downloads/uploads, failures may encounter for `docker push`
or `docker pull`.

This fix tries changes `maxDownloadConcurrency` and
`maxUploadConcurrency` to adjustable by passing
`--max-concurrent-uploads` and `--max-concurrent-downloads` to
`docker daemon` command.

The documentation related to docker daemon has been updated.

Additional test case have been added to cover the changes in this fix.

This fix fixes #20936. This fix fixes #22443.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

man/dockerd.8.md

@@ -44,6 +44,8 @@ dockerd - Enable daemon mode
 [**--log-driver**[=*json-file*]]
 [**--log-opt**[=*map[]*]]
 [**--mtu**[=*0*]]
+[**--max-concurrent-downloads**[=*3*]]
+[**--max-concurrent-uploads**[=*5*]]
 [**-p**|**--pidfile**[=*/var/run/docker.pid*]]
 [**--raw-logs**]
 [**--registry-mirror**[=*[]*]]
@@ -197,6 +199,12 @@ unix://[/path/to/socket] to use.
 **--mtu**=*0*
   Set the containers network mtu. Default is `0`.
 
+**--max-concurrent-downloads**=*3*
+  Set the max concurrent downloads for each pull. Default is `3`.
+
+**--max-concurrent-uploads**=*5*
+  Set the max concurrent uploads for each push. Default is `5`.
+
 **-p**, **--pidfile**=""
   Path to use for daemon PID file. Default is `/var/run/docker.pid`