Check for inconsistency between chain sync and block fetch on `MsgNoBlocks`

[dcoutts]

A block fetch server can in some circumstances give us a MsgNoBlocks.

In the client we need to:

1. make sure that we do not redundantly ask for the same blocks again in this case, (For mainnet 2.0)
2. check if the server is lying by checking that the points that were declared not available are not still on the chain of headers for this peer. (For mainnet 2.1)

[dcoutts]

Point 1 above is probably causing the test failure reported in #1435