peer networking performance bugs

[refacktor]

Testing PR #137 has taken a long time due to unrelated issues causing test nodes to often just hang and do nothing, sometimes crash, sometimes hang and then crash, etc:

• The old_heights in get_get_blocks_message only went back a maximum of 4096 blocks, this made it difficult to restart nodes after 5 days, because the fall back is to restart IBD from height=1
• There was a race condition in the call to start_sending(), sometimes this caused nodes to just hang
• The disconnect() logic didn't handle errors correctly, sometimes causing nodes to hang
• It was hard to tell the status of other peers and whether this node or other nodes were stuck
• The logic in ChainManager.step() was causing multiple IBDs to be initiated flooding the local peer and causing IBD to slow to a crawl
• Asking for more inventory at the end of inventory could hit a race condition, and cause IBD to restart from height=1, causing IBD to slow to a crawl

[refacktor]

These bugs have been fixed now in peer networking performance fixes. Ideally these changes would be a separate PR, unfortunately, they have only been tested together with the other changes in the PR.