[3.1 -> main] net_plugin fix startup issue when peer does not have requested blocks

[heifner]

Test failure of #115 caused by peer not requesting blocks when no blocks where received from the sync source peer.

Two things contributed to this.

1. The node was expecting blocks from a different source than the tracked sync_source because the sync_source was set to a different connection but no new request was sent. When the connection was closed that this node was expecting blocks from it did not request from a different connection because the sync_source indicated it was already syncing from a different connection. This caused it not to try a different connection on the close of the sync source connection.
2. This should have timed out, but since no blocks were ever received it never sent a new handshake.

Changed to keep sync_source up to date with with the connection actually syncing from. This is done by only updating sync_source when a request to sync from a peer is made.
Also changed to always send a handshake on a block heart beat timeout since there is no harm in sending an extra handshake.

Merges #135 to main.
Resolves #115