[Fix] Refactor srv_pipe to use queue-based architecture with ID dispatch

[Fix] Refactor srv_pipe to use queue-based architecture with ID dispatch

Replace per-request ev_io watchers with a single watcher using khash
for ID-based reply matching. This fixes potential deadlocks when multiple
commands are queued rapidly (e.g., during hyperscan compilation).

Changes:
- Add rspamd_srv_pipe_ctx with single watcher, send queue, and ID hash
- Make srv_pipe non-blocking on both ends with proper EAGAIN handling
- Add EAGAIN handling to main process write path
- Remove cache_dir from hs_loaded commands (available from config)