Add a filter to disable maintenance mode for all updates, everywhere.

[Zuige]

Currently there is no way for plugins to disable the wordpress maintenance mode from happening during updates done via the standard WP_Upgrader class.

I would like something like

add_filter( 'maintenance_mode', '__return_false' );

to just work. Which would completely stop WP_Upgrader::maintenance_mode from creating the .maintenance file.

The use case is for plugins that hook into the upgrade process, like ​https://wordpress.org/plugins/wp-safe-updates/ that want to either disable the maintenance phase, or replace it with something different.