Improve DB update commands

[schlessera]

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We currently have a command wp core update-db that sometimes needs to be run after an update of Core.

It would be great to have a mechanism to check whether an update is actually needed.

Also, both commands might make more sense as wp db ... commands, instead of wp core ... commands.

Suggestion:

wp db check-update     Checks for the need for WordPress database updates.
wp db update           Runs the WordPress database update procedure.

Related issue: https://github.com/wp-cli/wp-cli/issues/5085

cc @swissspidy