Implicit and explicit snapshots

[vweevers]

Implicit snapshots

Main task here is to fix the issue described in https://github.com/Level/leveldown/issues/796.

• [x] abstract-level (v2): https://github.com/Level/abstract-level/pull/54
• [x] classic-level (v2): create snapshot synchronously (https://github.com/Level/classic-level/pull/52)

Explicit snapshots

This is a new feature, explained by the first PR below. Prior discussions: https://github.com/Level/community/issues/45, https://github.com/Level/community/issues/47, https://github.com/Level/leveldown/issues/486.

• [x] abstract-level (3.0.0): https://github.com/Level/abstract-level/pull/42
• [x] level-supports (6.1.1): https://github.com/Level/supports/pull/32
• [x] abstract-level (3.0.0): https://github.com/Level/abstract-level/pull/93
• [x] memory-level (3.0.0): https://github.com/Level/memory-level/pull/16
• [x] classic-level (3.0.0: https://github.com/Level/classic-level/pull/110
• [ ] browser-level: document that explicit snapshots are not available
• [x] ~~level: document explicit snapshots~~ (documentation has moved)
• [ ] many-level: implement explicit snapshots
• [ ] rave-level: implement explicit snapshots