User and Sequential load strategies

[paulirwin]

This adds three new load strategies:

• SequentialLoadStrategy as an aggregate load strategy that executes multiple strategies in order
• ConstantUserLoadStrategy to attempt to hold a constant load based on how many existing tasks are still running
• SteppedUserLoadStrategy to attempt to hold a constant load at each step up or down

This also renames OneShot and Repeat user strategies to have Burst in their name, to better align with SteppedBurstLoadStrategy. This PR essentially creates three classes of strategies:

• Burst strategies enqueue a burst of tasks each iteration, regardless of how many are still running
• User strategies enqueue the delta number of tasks between the expected count at that iteration and the amount still running, to attempt to achieve an expected number of concurrent "users"
• and Meta strategies, like SequentialLoadStrategy, are a strategy-of-strategies.