[#964,!577] Added external socket ready handler to HAService

[#964,!577] Added external socket ready handler to HAService

src/hooks/dhcp/high_availability/ha_service.*
    HAService::socketReadyHandler(int tcp_native_fd) - new handler for external socket ready
    callback. It detects out-of-transaction ready socket and unregisters it.

src/lib/http/client.*
    isTransactionOngoing(int socket_fd) - new method to return true if any of the clients
    connections are using the socket in an ongoing transaction