Tracking issue: implicit and explicit snapshots

[Vincent Weevers]

Implicit snapshots

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

• abstract-level (v2): Level/abstract-level#54
• classic-level (v2): create snapshot synchronously (Level/classic-level#52)

Explicit snapshots

This is a new feature. Prior discussions: #45, #47, Level/leveldown#486.

• abstract-level (semver-minor): Level/abstract-level#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