Feature Request: Add ability to restore TabletType on restart based on topology.

[sbaker617]

Feature Description

When an existing tablet restarts, it always comes up as the type specified by --init_tablet_type, this can make maintenance and upgrades more challenging or lead to unexpected behavior with vttable when services restart who's TabletType have changed external to the service definition.

Use Case(s)

• Upgrading vttablet binaries, being able to restart and have types persist.
• allow all tablets to initially deploy as SPARE, with types changed to REPLICA/RDONLY as needed and have types persist on service restarts without edits & reload of .service files.