Special oplog implementation for ephemeral workers

[vigoo]

Resolves #935

Introduces a new oplog implementation for ephemeral workers, with the following properties:

• It directly writes to the last oplog archive (targeting the blob store with the default config)
• It has a separate "max entries before commit" configuration, which is bigger by default than the original one
• In many cases where there is a forced commit for durable workers (for example after remote writes), it does not commit
• Even if it commits, it does that in the background, except in some "forced commit" situations.

[noise64]

is this also async / no guartantees? just thinking about the naming

[vigoo]

For durable workers all commits are as before - they are "async blocking" until the entries are written.
For ephemeral workers, the three levels are:

• like the above
• commit, but on a spawned background fiber
• ignore

tried to bring in names that makes sense, maybe failed :)