Introduce a command to close all or a subset of connections

[michaelklishin]

Similar to the idea in rabbitmq/rabbitmq-server#817 but to force close all active connections, in a particular vhost or (worth considering) globally. This is sometimes useful for technical ops.

This will be a good feature to introduce now that rabbitmq/rabbitmq-server#500 is finished and merged.

More ideas suggested by @johnfoldager in the original issue in rabbitmq-server:

• a per-connection delay to avoid/lessen the probability of causing a thundering herd, e.g. --per-connection-delay in milliseconds
• a --global flag that would close all active connections to a node (across all vhosts)
• a --limit of how many connections are closed (cannot work with --global, only per-vhost)