🔒 Fix DoS vulnerability in Credit Protocol contract

`#executeUcacTx` is an inexpensive and unauthenticated function that
increases the transaction counter for a UCAC without actually
performing a transaction, up to that UCAC's transaction limit.

A motivated attacker can call this function continuously to perform
a denial-of-service (DoS) attack on a UCAC, preventing legitimate
transactions using that UCAC from being processed.

Once started, this attack can be maintained indefinitely.

Potential mitigations/effects on a live contract include:

 * An attack on a UCAC may incentivize its stakeholders to unstake
   their tokens.

 * The fewer tokens staked in a UCAC, the less expensive the attack
   becomes to perform and maintain.

 * Staking more tokens in the UCAC will increase the transaction limit,
   thus increasing the cost to perform and maintain the attack.

The best mitigation for this attack, then, for a live contract, would
be for token holders to stake enough tokens in the affected UCAC to
make the attack too expensive for the attacker to maintain.

contracts/CreditProtocol.sol

@@ -105,7 +105,7 @@ contract CreditProtocol is Ownable {
         return adjustedTxLevel;
     }
 
-    function executeUcacTx(address _ucacContractAddr) public {
+    function executeUcacTx(address _ucacContractAddr) private {
         uint256 txLevelBeforeCurrentTx = currentTxLevel(_ucacContractAddr);
         uint256 txLevelAfterCurrentTx = txLevelBeforeCurrentTx + 10 ** 27 / txPerGigaTokenPerHour;
         require(ucacs[_ucacContractAddr].totalStakedTokens >= txLevelAfterCurrentTx);