tabletmanager: Set externally reparented time on master during InitShardMaster.
tabletmanager: Set externally reparented time on master during InitShardMaster.

After this change, masters promoted by vtctl InitShardMaster will publish a non-zero value for TabletExternallyReparentedTimestamp on the healthcheck streaming connection.

This is necessary for the vtgate buffer during tests. Without it, the
buffer cannot tell wether a shard was just initialized or already
reparented. Only in the second case, it can use the reparented timestamp
to check if the last failover was too recent.

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