Implicit and explicit snapshots

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, 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.

• abstract-level (3.0.0): https://github.com/Level/abstract-level/pull/42
• level-supports (6.1.1): https://github.com/Level/supports/pull/32
• abstract-level (3.0.0): https://github.com/Level/abstract-level/pull/93
• memory-level (3.0.0): https://github.com/Level/memory-level/pull/16
• classic-level (3.0.0: https://github.com/Level/classic-level/pull/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

Read the linked abstract-level pull requests and prior community discussions first to understand the explicit snapshot contract. Then inspect browser-level, many-level, and rave-level; document browser-level's limitation and implement explicit snapshots in the two remaining packages. Done means all unchecked items are completed and the package behavior matches the established snapshot API.