Fix long overflow in Block.distributeBlockRewardToQoraHolders()

Sadly no native 128bit integer support in Java 11 so resorting to using
BigInteger.

Added/improved unit tests to cover.

src/test/resources/test-settings-v2-qora-holder-extremes.json

@@ -0,0 +1,7 @@
+{
+  "restrictedApi": false,
+  "blockchainConfig": "src/test/resources/test-chain-v2-qora-holder.json",
+  "wipeUnconfirmedOnStart": false,
+  "testNtpOffset": 0,
+  "minPeers": 0
+}