Upstream keepalive: clean read delayed flag in stored connections. If a connection with the read delayed flag set was stored in the keepalive cache, and after picking it from the cache a read timer was set on that connection, this timer was considered a delay timer rather than a socket read event timer as expected. The latter timeout is usually much longer than the former, which caused a significant delay in request processing. The issue manifested itself with proxy_limit_rate and upstream keepalive enabled and exists since 973ee2276300 (1.7.7) when proxy_limit_rate was introduced.