* fix: p2p sync with lagging peer
no need to sync with lagging peer, which could make the local chain stalling as well.
* fix: do not drop lagging peer, will retry it later.
The lagging peer is probably already the best peer with largest total difficulty.
Shoule not remove it, since p2p is a bidirectional connection, drop it could make
the peer unable to sync with this peer as well.
And the lagging peer could catch up later, so keep it.
* p2p: add lagging field in Peer
lagging peer will be connected, but won't be used to sync.
the lagging flag can be clear once the Peer updates its latest block state.
* test: fix UT compile issue
* fix: lagging peer func rename
* test: fix a UT fail of download test
errStallingPeer is replaced by errLaggingPeer in this case
* fix: lagging issue in light mode
* test: add and resolve UT of lagging peer
b3a20c7611