inventory lists sent with the objects with the most proof of work first

Modified code so that clients send out their inventory lists with the objects with the most proof of work first. The purpose is to ensure that a small minority of users contributing a relatively low amount of computational resources will not be able to DOS the network because their low proof of work messages are likely to be the last ones delivered and the first ones delayed past object expiration, as discussed here: https://bitmessage.org/forum/index.php?topic=4067.150

For security reasons, receiving clients still request objects in random order, but when the network is overloaded it is the low POW objects that are much more likely to be dropped by inv message splitting and/or flooding mitigation. However, if this "supply side" solution isn't enough then objects can be requested in FIFO order to ensure that high POW messages have a strictly higher priority (rather than just a statistical advantage).