Enforce batch size, add configuration options.

[afck]

There are different ways one may want to specify the batch size in Honey Badger (see the TODO):

• Currently the batch size itself is a parameter; each node proposes batch_size / num_nodes transactions per epoch. This is desirable e.g. in a blockchain with a fixed maximum block size.
• To achieve its asymptotic complexity, the paper suggests a batch size of lambda * num_nodes * num_nodes * log(num_nodes). Since this comes at the cost of a quickly increasing latency, it's not always the right choice in practice.
• There are other options in between with varying tradeoffs, e.g. lambda * num_nodes (constant proposal size per node) or lambda * num_nodes * log(num_nodes), etc.

We could make the batch size specification an enum that allows picking one of these options. This is particularly important for DynamicHoneyBadger, where the number of nodes can change.

Also, we should enforce the limit: If another node is sending us a proposal that exceeds the per-node allowance, the transfer needs to be interrupted, the proposal ignored and the node reported/logged.

[afck]

~With #104, this should only affect the implementation of the default queue, not Honey Badger.~

Edit: ~…and of Broadcast, because that's the point where we need to enforce the message size limit.~ Actually, the user should refuse to receive messages beyond a certain size. We need to communicate or document that size somehow, though.

Edit 2: It does affect Queueing Honey Badger, of course. If we receive a contribution with too many transactions, we need to report a fault.