implement aboutToFinish for persist thread to allow final save before shutdown
implement persist backup
add persist settings to domain settings, make default backup interval 30 minutes
OctreePersistThread defaults to no backup, server always uses settings
Merge branch 'master' of https://github.com/highfidelity/hifi into persistThreadImprovements
winblows
Merge pull request #3798 from ZappoMan/persistThreadImprovements

Persist thread improvements/Backup support