Prevent streaming API denial-of-service (resolves #1019)

What does this MR do?

Patches multiple potential DoS vectors in the streaming API:

• Use a bucket limiter instead of sliding-scale.
• Limit the number of active connections per client.
• Automatically close the connection when a rate limit is hit.
• Automatically evict note cache when a connection closes.
• Automatically cancel note subscriptions when a connection closes.
• Limit the number of note subscriptions to 256 per connection. Excess subscriptions will be automatically canceled in LIFO order.
• Limit UPGRADE requests by user ID instead of request IP, when applicable.
• Fix Channel leak when two channels share the same ID.
• Optimize storage, index, and lookup for cached notes and channels.

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