EX-6373 pcc: asynchronous PCCRO attach command support

EX-6373 pcc: asynchronous PCCRO attach command support

Currently PCCRO attach via the command "lfs pcc attach" will block
during the data copying.
There is a requirement that this command can also do data copy
asynchronously. Thus we add an option "--async|-A" to the command
which will not block while the file data is being fetched.

Add sanity-pcc/test_{103, 104} to verify that it works correctly.

Change-Id: I6f31190c8b9e9b9876b34f8e484c6c8b7f16b6db
Signed-off-by: Qian Yingjin <qian@ddn.com>
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/49133
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Feng Lei <flei@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>