refactor(block-producer): order batches within a block

[Mirko-von-Leipzig]

As mentioned in this comment, this PR makes it clear that batches within a block have a given order. This intention signaling is improved by using a vector instead of a map (with accidental ordering). As alluded to in this comment from #397.

The current batch selection strategy just selects an arbitrary available root batch. A root batch by definition is one who's parents have already been selected previously; or in other words dependency order is enforced within the selection process.

The set of batches is then consumed in order by the block builder. Though it does use a BTreeMap which makes this a bit opaque (the keys form a sort of FIFO so ordering is kept), and I guess more or less implies that the ordering is accidental and not necessarily intentional.

This issue is technically complete but I'll create a PR changing the BTreeMap to a Vec so that this ordering is made more clear.

Closes #397.

[bobbinth]

Looks good! Thank you! I left a couple of small comments inline.