Async media support: MSC2246

[turt2live]

Original PR: #364

Functionality:

• POST /_matrix/media/v1/create
• Per-user quotas
• Concurrency limit
• Actually check media table for duplicate media IDs during generation (and un-reserve on error)
• Claim the ID (and un-reserve on upload)
• Download/thumbnail timeouts
• Clean up dead records via background task