Delete list of keys in batch

[tleish]

SUMMARY

This PR updates the existing Kredis::Migration#delete_all method to:

• Allow a key pattern OR array passed to delete keys
• Add logging for #delete_all

DETAILS

There's occasion where you might want to clear a large number of keys, but using a pattern creates too broad of a stroke.

Today, you would have to run:

found_users.each { |user| user.my_kredis_key.clear }

A much more efficient method would be to delete a list of keys in batch rather than separately:

Kredis.redis.del *found_users.map { |user| user.my_kredis_key.key }

This change add DSL to the process and logging of the event:

Kredis::Migration.delete_all *found_users.map { |user| user.my_kredis_key.key }