test: raft: randomized_nemesis_test: environment: implement server `stop` and `crash`
test: raft: randomized_nemesis_test: environment: implement server `stop` and `crash`

`stop` gracefully stops a running server, `crash` immediately "removes" it
(from the point of view of the rest of the environment).

We cannot simply destroy a running server. Read the comments in `crash`
to understand how it's implemented.

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla