Make possible to copy-back ibdata1 in multi-thread

[Shahriyar Rzayev]

During backup it is clear that ibdata1 backed-up in multi-thread

xtrabackup: Starting 10 threads for parallel data files transfer
171127 14:08:56 [03] Compressing, encrypting and streaming ./ibdata1

But for copy-back it is copied in main thread.

171127 14:14:51 [01] Copying ib_logfile0 to /home/shahriyar.rzaev/XB_TEST/server_dir/PS231117-5.7.20-18-linux-x86_64-debug/node2/ib_logfile0
171127 14:15:03 [01] ...done
171127 14:15:04 [01] Copying ib_logfile1 to /home/shahriyar.rzaev/XB_TEST/server_dir/PS231117-5.7.20-18-linux-x86_64-debug/node2/ib_logfile1
171127 14:15:18 [01] ...done
171127 14:15:19 [01] Copying ibdata1 to /home/shahriyar.rzaev/XB_TEST/server_dir/PS231117-5.7.20-18-linux-x86_64-debug/node2/ibdata1
171127 14:15:19 [01] ...done
xtrabackup: Starting 10 threads for parallel data files transfer
171127 14:15:19 [03] Copying ./xtrabackup_slave_info to /home/shahriyar.rzaev/XB_TEST/server_dir/PS231117-5.7.20-18-linux-x86_64-debug/node2/xtrabackup_slave_info

Maybe it will be possible to change this main thread copying for --copy-back --parallel as well.

[Shahriyar Rzayev (rzayev-sehriyar)]

Percona now uses JIRA for bug reports so this bug report is migrated to: https://jira.percona.com/browse/PXB-795