audit: Comp Timelock Doesn't Support Duplicate MetaTransaction Queueing

[Orland0x]

The execute function of the contract fails silently when trying to queue the same MetaTransaction
twice within the same proposal, due to the internal workings of the Compound Timelock contract.
If two identical MetaTransactions are present in the payload, the two queueing attempts will only be counted as one in Compound's Timelock. This can cause seemingly valid proposals to fail when executed after the timelock delay has elapsed.

[Orland0x]

We have a salt inside the meta tx for this reason.

[pscott]

FIX:

Ensure there are not duplicate MetaTransaction (excluding the salt) by using an array in memory (complexity is O(n^2) but we don't expect the list to be long in the average scenario: and it's better to use an array in memory than a mapping in storage for the "normal" cases)