Benchmark: Support "hot" accounts

[sentientwaffle]

Add two new flags to the benchmark:

• --account-count-hot (default: 0)
• --transfer-hot-percent (default: 100)

So, the benchmark's default behavior is unchanged (since there are no hot accounts). But if --account-count-hot=N, then N of all of the (10k) accounts are "hot" -- --transfer-hot-percent (100% by default) of transfers pick one of the hot accounts as their debit account.

Note that this won't have much impact on throughput with only 10k accounts total, since a single table can fit almost 260k accounts.