Limit number of packets per transaction

[webmaster128]

If there are many (like 100) packets to be relayed (result of getPendingPackets), they are all sent in one transaction (in receivePackets). The auto-gas simulation then fails once the simulation hits the block gas limit.

[ethanfrey]

Ah, yes, we definitely need to break them into groups.
Also handle the case when one of them fails or uses too much gas (this will happen more with ICA), such that we don't keep trying it and blocking others.

A lot of these edges to make it robust enough for the wild.