Wait for HTTP/2 requests to finish

[samuel-williams-shopify]

Summary

• own concurrent HTTP/2 request handlers with an Async::Barrier
• wait for active handlers before Server#each returns
• stop remaining handlers if request enumeration exits exceptionally
• cover queue shutdown while a handler is still active

Testing

• bundle exec sus test/async/http/protocol/http2/server.rb
• bundle exec sus (243 passed, 3 skipped)
• bundle exec rubocop (128 files, no offenses)

This is stacked on #234 because closing the request queue defines when handler draining begins.