Database utilities [#3512]

• Closes #3512 (closed).
• Implements a remote key-value store protocol.
• Adds ForEach(func(*record.Key, []byte) error) error to keyvalue.Store.
• Defines a scheme for database URLs that supports remote databases and snapshots.
• Implements a clone command to clone a database.
• Implements a sync command to synchronize a target database to a source database.
• Generalizes debug badger ls to debug db analyze (can analyze an arbitrary key-value store).
• These commands accept database URLs and can analyze/sync from/clone from a remote database or snapshot.
• Exports internal/util/indexing.File to facilitate other kinds of indexing (since memory-mapped files are fast).

Changelog: feature