Implement a log-like storage governed by an embedded database

[ornamental]

Requirements

1. The storage must support any number of concurrent readers and one writer not blocked by the readers.
2. Each reader must observe the snapshot of the storage relevant at the moment of its creation; it must not see the changes added afterwards, either committed or not.
3. The writer must observe the most relevant state with its own uncommitted changes.
4. The writer must support commit and rollback operations.
5. However much data a writing transaction might supply, the RAM footprint must not grow proportionally.
6. The storage must be able to transfer data from byte arrays, input streams, and readable byte channels.