Add backup and restore with timetable

[narendly]

This is a merge-able public version of https://github.com/apache/zookeeper/pull/1492 contributed by @hanm .

Some notable features are

• uses a file system based storage (NFS implementation included)
• uses generic, public libraries
• adds a restore feature using Timetable

Also available as in LinkedIn's public fork: It is currently publicly available here in LinkedIn's public fork of ZooKeeper: https://github.com/linkedin/zookeeper/commits/li-dev/release-3.6.2-1-backup (a few changes made to make it mergeable)

[eolivelli]

@narendly can you please advertise this great work on [email protected] ?

[narendly]

@eolivelli Will do - thanks!

[eolivelli]

What's the status of this patch ?

[eolivelli]

closed/reopened in order to trigger CI