Tracking issue: implicit and explicit snapshots

[vweevers]

Implicit snapshots

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

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

Explicit snapshots

This is a new feature. Prior discussions: https://github.com/Level/community/issues/45, https://github.com/Level/community/issues/47, https://github.com/Level/leveldown/issues/486.

• [ ] abstract-level (semver-minor): https://github.com/Level/abstract-level/pull/42
• [ ] level-supports (semver-minor): add explicitSnapshots property
• [ ] abstract-level (semver-minor): implement explicit snapshots
• [ ] memory-level (semver-minor): implement explicit snapshots (can just take a reference to the RB tree)
• [ ] classic-level (semver-minor): implement explicit snapshots
• [ ] browser-level (semver-patch): document that explicit snapshots are not available
• [ ] level (semver-patch): document explicit snapshots
• [ ] many-level (semver-minor): implement explicit snapshots
• [ ] rave-level (semver-minor): implement explicit snapshots