Add a BlockFetch leashing attack

[Niols]

This attack is only triggered if one replaces FetchModeDeadline by FetchModeBulkSync and reduces the bfcMaxConcurrency* constant to be lower or equal to the number of adversaries. (Typically, in the upcoming BlockFetch change, bfcMaxConcurrencyBulkSync will be 1.)

[Niols]

Note that tests fail, as discussed on Tweag's Slack.

[amesgen]

Superseded by #1179