RFC: vttablet breaking changes

[sougou]

This is an expansion of #6131 to mainly highlight some changes that are not backward compatible. These decisions were based on our perception of how vitess is currently used, with the hope that they should not materially affect any existing deployments.

• init parameters will become mandatory: init_keyspace, init_shard and init_tablet_type will be made mandatory. The flow where you could issue a vtctlclient InitTablet and then invoke vttablet without those init parameters will not be supported.
• Consequently vtctl's InitTablet and UpdateTabletAddrs will also be deprecated.
• demote_master_type is deprecated: Now that init_tablet_type is mandatory, that value can act as the demote_master_type. There is no need for a separate flag.
• VTTablet will publish the discovered mysql port on startup, and will not update it during the life of the process. There is no known use case where a mysql port is expected to change. So, there is no need to keep polling mysql for its port.
• Decouple enable_replication_watcher from disable_active_reparaents. Currently, vttablet fixes replication only if enable_replication_watcher is true and disable_active_reparents is false. We will change this behavior to depend only on the disable_active_reparents flag.

[sougou]

It turns out that demote_master_type is still used by a few people. I'll bring the flag back into existence to allow for transition. The value will be required to match init_tablet_type. We can sunset the flag after one release.

[sougou]

The following were undocumented, and likely unused behaviors. These are changing in #6461:

• Transitioning from a serving type to SPARE would go through a brief lameduck period specified by the serving_state_grace_period (default 0). This has been removed. The transition will now happen immediately even if a grace period is specified.
• It was possible to enable health reporting and heartbeat at the same time. Performing the two at the same time was not useful. Wit the new change, enabling heartbeat automatically disables the polling mode of checking for replica health.
• The ACL rules used to be reported on the vttablet status page. The design was using an incorrect framework. This has been removed.

[deepthi]

Fixed in #6461