Looks really interesting. I assume this suggests that ClickHouse is going to gradually switch to using this library for network and I/O, thus addressing the main weakness (in my mind) of C++ thread-per-connection servers, which is, they (surprisingly!) create too many threads and can't really handle more than, say, a thousand active connections at the same time. It mostly matters for async INSERTs in this case of course, not for SELECTs, although generally it applies to both.
[0] https://github.com/scylladb/seastar