Impose minimum and maximum lot sizes

This to avoid signer DoS (for the minimum 0.0005 BTC lot size) and to
avoid putting more funds at risk than reasonable in a single deposit
(for the maximum 10 BTC lot size).

solidity/contracts/system/TBTCSystem.sol

@@ -185,16 +185,27 @@ contract TBTCSystem is Ownable, ITBTCSystem, DepositLog {
     function beginLotSizesUpdate(uint64[] calldata _lotSizes)
         external onlyOwner
     {
-        for( uint i = 0; i < _lotSizes.length; i++){
-            if (_lotSizes[i] == 10**8){
-                lotSizesSatoshis = _lotSizes;
-                emit LotSizesUpdateStarted(_lotSizes, block.timestamp);
-                newLotSizesSatoshis = _lotSizes;
-                lotSizesChangeInitiated = block.timestamp;
-                return;
+        bool hasSingleBitcoin = false;
+        for (uint i = 0; i < _lotSizes.length; i++) {
+            if (_lotSizes[i] == 10**8) {
+                hasSingleBitcoin = true;
+            } else if (_lotSizes[i] < 50 * 10**3) {
+                // Failed the minimum requirement, break on out.
+                revert("Lot sizes less than 0.0005 BTC are not allowed");
+            } else if (_lotSizes[i] > 10 * 10**8) {
+                // Failed the maximum requirement, break on out.
+                revert("Lot sizes greater than 10 BTC are not allowed");
             }
+
+            newLotSizesSatoshis = _lotSizes[i];
         }
-        revert("Lot size array must always contain 1BTC");
+
+        require(hasSingleBitcoin, "Lot size array must always contain 1BTC");
+
+        lotSizesSatoshis = _lotSizes;
+        emit LotSizesUpdateStarted(_lotSizes, block.timestamp);
+        newLotSizesSatoshis = _lotSizes;
+        lotSizesChangeInitiated = block.timestamp;
     }
 
     /// @notice Set the system collateralization levels