Merge IPv4 and IPv6 addresses into a single type (#32)

bpf/flow.h

bpf/flows.c

pkg/agent/agent_test.go

pkg/ebpf/bpf_bpfeb.o

pkg/ebpf/bpf_bpfel.o

pkg/exporter/grpc_proto.go

pkg/flow/account_test.go

pkg/flow/record.go

pkg/flow/record_test.go