Implicit and explicit snapshots

[vweevers]

Implicit snapshots

Main task here is to fix the issue described in Level/leveldown#796.

• abstract-level (v2): Breaking: require snapshots to be created synchronously abstract-level#54
• classic-level (v2): create snapshot synchronously (Breaking: bump abstract-level to 2.0.0 classic-level#52)

Explicit snapshots

This is a new feature, explained by the first PR below. Prior discussions: #45, #47, Level/leveldown#486.

• abstract-level (3.0.0): Add explicit snapshots to README abstract-level#42
• level-supports (6.1.1): Add implicit- and explicitSnapshots supports#32
• abstract-level (3.0.0): Implement explicit snapshots abstract-level#93
• memory-level (3.0.0): Implement explicit snapshots memory-level#16
• classic-level (3.0.0: Implement explicit snapshots classic-level#110
• browser-level: document that explicit snapshots are not available
• level: document explicit snapshots (documentation has moved)
• many-level: implement explicit snapshots
• rave-level: implement explicit snapshots