Feathercoin Core 0.21.0

Merge pull request #396 from wellenreiter01/patch-QPainterPath

fix compile problem with Qt5

checkpointsync reduce holding of cs_main

Hold cs_main for all uses of mapBlockIndex

Hold cs_main for all uses of mapBlockIndex

Checkpoint sync switch client to checkpointed chain

Checkpoint sync switch client to checkpointed chain

Merge pull request #386 from FeatherCoin/0.18

Remove QtCreator generated file

Merge pull request #383 from FeatherCoin/0.18

Testnet seeder and stale tip changes