UTXO DoS Vulnerability

[StephenButtolph]

Describe the bug
Inside our UTXO based VMs, we map addresses to UTXOs that reference the address. However, these UTXOs are stored in a single list for each account. Someone can DoS us by sending many UTXOs to the same address, performing an O(total number of UTXOs attached to address) operation per commit of every tx.

To Reproduce
Run a throughput test issuing transactions to only one address.

Expected behavior
The time to execute a transaction should be O(number of objects in the transactions).

Operating System
Universally applicable.