Reserve a section of the swingstore for the host

[mhofman]

What is the Problem Being Solved?

Currently host cosmic-swingset uses the key cosmos/meta of the swingstore to save it's own state just before committing. We'd like to expand what is currently stored by the host, and keep it more structured.

supports #5350

Description of the Design

Reserve the prefix host. for host operations. Swingset should not attempt to read or write in those values.

Security Considerations

It's probably sufficient to have this simply a documented contract between the host and Swingset, without explicit enforcement. The writes performed by the host are already not part of the crank buffer or activityhash computation performed by swingset, but we could add a check there for good measure.

Test Plan

TBD