Defer retirement of active multiplexed resources

[samuel-williams-shopify]

Summary

• decrement a resource's usage before deciding whether it can be reused
• remove non-reusable resources from availability immediately, while retaining active resources until their final release
• keep non-viable active resources counted toward the pool limit and preserve forceful explicit retirement

This provides the pool ownership semantics needed by socketry/async-http#245 without adding HTTP-specific graceful shutdown behavior to the pool.

Tests

• bundle exec sus
• git ls-files -z '*.rb' | xargs -0 bundle exec rubocop
• async-http HTTP/2 GOAWAY and reset regressions against local async-pool and protocol-http2 branches