feat(validations): invert "unstake-it-all" logic

namely, use 0 instead of u64::MAX as the "unstake-it-all" meta-value

Author: aesedepece

validations/src/validations.rs

@@ -1520,7 +1520,9 @@ pub fn validate_unstake_transaction<'a>(
     // Introduced in V2_1:
     // Apply a protocol-level "rule of convenience", where an amount to unstake of u64::MAX actually
     // means "unstake all my stake".
-    if protocol_version >= ProtocolVersion::V2_1 && amount_to_unstake == u64::MAX {
+    // Please take note that this validation sctrictly needs to take place before checking whether
+    // the "remainder" amount meets the minimum stake. Otherwise, we would never reach this block.
+    if protocol_version >= ProtocolVersion::V2_1 && amount_to_unstake == 0 {
         amount_to_unstake = staked_amount;
     }