v0.5.0-rc4

Fixes for issues found while testing v0.5.0-rc3.

• Fixed querying posts by refs and tags
• Fixed support for large number of websockets.

What's Changed

• Fix: filter amended posts by ref on /api/v0/posts.json by @odesenfans in #417
• Fix: filtering by tags by @odesenfans in #416
• Fix: increase POST /messages sync timeout by @odesenfans in #418
• Fix: restrict /pubsub/pub topic to the message topic by @odesenfans in #419
• Fix: support very large number of open websockets by @odesenfans in #422

Full Changelog: v0.5.0-rc3...v0.5.0-rc4