patch01: address path-based groupmint

[benjaminbollen]

problem: for path-based groupmint (much like with external groupMint) the minted group tokens were delivered to the sender. In the verification of the flow matrix of a path transfer, this resulted in any groupmint always netting out zero, and as such always being allowed for the maximum balance of the avatar - IF the avatar had opted out of consented flow for any (malicious) group.

In this patch, the group tokens that are minted over the path transfer are minted in-place onto the group address - so the path transfer needs to explicitly move the group tokens onward (or the group is set as the final receiver in the stream).

This patch only addresses that now the amount "mintable"/"stealable" is limited to the total capacity of other Circles that the avatar is able to receive from their trusted Circles (in exchange for the tokens minted into the group). So the core problem remains that, without consented flow, or when an avatar opts out from consented flow, an attacker can steal valuable Circles from a user who opts-out of consented flow - albeit at a possibly rate limited flow (which is still unacceptable).

It is important to note that, while this patch by itself alone does not address the attack vector (the main one remains: being able to opt out from consented flow), it is well-worth considering this patch because this implementation fix for path-based group mint is the more logical, and elegant one.

[benjaminbollen]

included in #42