Continuous ingest

[philrz]

The following text was present in a retired "lake design" document (see #3803). It has been established that this was really a pending to-do, so this issue tracks its ultimate implementation and corresponding updates to docs.

## Continuous Ingest

While the description above is very batch oriented, the Zed lake design is
intended to perform scalably for continuous streaming applications.  In this
approach, many small commits may be continuously executed as data arrives and
after each commit, the data is immediately readable.

To handle this use case, the _journal_ of branch commits is designed
to scale to arbitrarily large footprints as described earlier.

Note some existing things that point to the current state of affairs as regards this functionality:

• "Live" data import with Logstash #3151
• https://zed.brimdata.io/docs/next/commands/zed#210-merge
• lake manager #3923