Cache IEEE (Long) Address to and from Network (Short) Address

[Frans-Willem]

For now we're calling UTIL_ADDRMGR_NWK_ADDR_LOOKUP and UTIL_ADDRMGR_EXT_ADDR_LOOKUP whenever we need to convert between the two, but that's sending a lot of data over the serial line.
Ideally we should cache this and not call the USB stick all the time.