MEDIUM: server: implement "purge all servers" CLI

MEDIUM: server: implement "purge all servers" CLI

Implement a new CLI command "purge all servers <be>". It tries to delete
every server present in a backend or if not possible flag them for
background deletion. If the deletion is impossible due to a specific
configuration of the server, the instance is skipped and the following
entries are handled.

At the end of the iteration, a summary is displayed on the CLI with the
number of servers deleted, flagged and skipped. If at least one server
is flagged for background deletion, proxy purge task is scheduled.

The command runs via a parser and a handler. While it is not strictly
necessary currently, this will serve as a skeleton to implement
yielding.