When a vector store node becomes unreachable, a client request sent
before the keep-alive timer fires would hang until the CQL query
timeout was reached.
This occurred because the HTTP request writes to the TCP buffer and then
waits for a response. While data is in the buffer, TCP retransmissions
prevent the keep-alive timer from detecting the dead connection.
This patch resolves the issue by setting the `TCP_USER_TIMEOUT` socket
option, which applies an effective timeout to TCP retransmissions,
allowing the connection to fail faster.
Closesscylladb/scylladb#27388
a54bf50290