GitLab

Also update ZoneTableSegmentMapped testcases to test the created segment
as a base ZoneTableSegment.

- include style ("" vs <>) per consensus
- null pointer comparison style
- documentation wording fix

as a zone writer can only update a single zone.

as it's not only used for reloading (or not even for "loading").

This also makes ZoneWriter a concrete class (and removes
ZoneWriterLocal).

Clarify high-level design of IPC

it's a leftover of an intermediate attempt of testing the case; on a closer
look, testing send_command turned out to be better and sufficient.