Add hotswaps

[mafintosh]

No description provided.

[HDegroote]

Sanity check (quicker than fully tracing it myself):

hypercore/lib/replicator.js

Lines 1973 to 1977 in 69d15e9

	_clearRequest (peer, req) {
	if (req.block !== null) {
	this._clearInflightBlock(this._blocks, req)
	this._unmarkInflight(req.block.index)
	}

=> Do we not need to remove the hotswap here, and/or handle the case where this is the answer to a hotswap request?

[mafintosh]

Checking

[mafintosh]

We do, nice one

[mafintosh]

@HDegroote pushed a tweak for your thing above