[server] Implement cross-library copy and move.
Do cross library copy/move in background.
[server] Tool to migrate repo to new layout.
[server] Fix bugs in copy/move.
Fix a few bugs.

* Sort fields in fs JSON object.
* Fix a minor bug in clone.
* Allow to use symlink for seafile-data directory.
* Enlarge sqlite connection pool and allow setting the size.
* Ignore invalid UTF-8 filename.
Improve copy task API.
Improve seaf-migrate tool.
Fix a minor bug in fsck.