Treat a declined kvdb-to-SQL migration prompt as a startup-abort
condition that shuts litd down fully. This prevents litd from leaving
the status server running after the operator refuses the migration
prompt.
The reason why this is motivated, is that `litd` at this stage of the
startup process will not be ready to handle an `litcli stop` RPC call.
Add a config option and env variable which allows users to skip the
migration confirmation prompt. This is useful for users who which are
running in a non-interactive environment, and does not have access to
pass input to stdin.
Prompt before automatically migrating legacy kvdb state to SQL when
litd starts with a SQL backend and active bbolt data is still present.
Detect prior migrations by checking for the SQL tombstone marker so
already-migrated stores can start without prompting. Add unit coverage
for the prompt flow and wire stdin through the itest harness so the
migration restart path can acknowledge the prompt automatically.
