Emit an extension event when messages are deleted, wire in to other parts of Inbucket

• Consolidate message metadata types
• Add event type
• Emit from file store
• Emit from mem store
• Emit UI event
• REST event
• Expiration event
• Lua binding
• Remove deleted messages from monitor: Monitor shows a message after its been deleted #181
• Mailbox purge support
• Maintain v1 socket API support
• Confirm ring buffer does not shrink on delete