[push] Add synchronized update strategy

[nemesifier]

We need a strategy that allows pushing updates that will be executed at a certain time in the future.

This would allow to push configurations that can break a network (eg: change the channel or routing deamon of a mesh network) in a relatively safe way, because the changes will be executed at the same time on all the devices, if anything goes wrong, openwisp-config will rollback the configuration to the previous state.

By default the reload of the configuration on the device should happen after 5 minutes from the change, we shall call this as the reload_config_datetime for the sake of explaining. This delay should be configurable.

The strategy shall be designed only for OpenWRT devices which have openwisp-config installed on board. It should consist of the following steps:

• connect to the device
• stop the openwisp-config deamon to avoid it from downloading and applying the configuration by itself
• modify the openwisp-config daemon configuration in order to set the date of the upgrade, save the change, sleep for a number of seconds randomized between 0 and 30 (to avoid overloading the controller, the value of 30 should be configurable and default to 10% of the delay, hence 30 seconds), then start the deamon again
• the deamon will then download the configuration but wait to apply it until the reload_config_datetime, once the configuration is applied we must add a way for the agent to resume working normally, maybe this configuration can be cleared out or another configuration key can be added, eg: reload_config_datetime_enabled which can be set to False

Related to openwisp/openwisp-config#104