[5.0] P2P: Fix: Throttling of last block of request caused lost block

[heifner]

When throttling was enabled on a connection, the net_plugin would assume the last block of a peer_requested was sent and reset the peer_requested indicating no other sync blocks need to be sent to the peer. This caused the block to not be sent or rescheduled to be sent later resulting in the peer not receiving the block. The peer would have to wait for a full timeout before requesting the required block from a different peer.

Resolves #1776

[greg7mdp]

I think I'd get rid of num, and use cc.fetch_block_by_number( peer_requested->last + 1 ); and peer_requested->last below, but it is a matter of preference.