Add keepalive feature. (#592)

* Add keepalive feature.

This allows a debugger to request that a given hart stays available for
debugging (e.g. is not powered down). Fixes #519.

* Remove keepalive status reading.

* writes -> sets

Co-authored-by: Ernie Edgar <43148441+ernie-sifive@users.noreply.github.com>

* Make the example more conservative.

Feedback from Robert Chyla.

Co-authored-by: Ernie Edgar <43148441+ernie-sifive@users.noreply.github.com>