[Hammer] Monitor leaf integration

Log external APIs reference leaves at four different points:

1. Add requests: data clients send
2. Add responses: the index at which the leaf will eventually be integrated
3. Leaf bundles: where leaves contain the data sent by clients
4. L0 tiles: listing hashes of leaves included in leaf bundles

Today, the conformance tool / hammer does not check that these four match with each other. For instance:

• A request to add data X, could return index n, and the entry later integrated at index n might not match
  data X
• The leaf at index p, might not match with the hash included in the tile at the corresponding position

After speaking with both @roger2hk and @mhutchinson about this, I'm not 100% convinced that the hammer in its current form would allow to implements {all, any of, some of} these checks, but let's at least discuss options.
Some of these can be implemented with integration tests (like other checks the hammer runs), but implementing them with a conformance binary supporting heavy load could ensure that a log implementation doesn't go off track, and that all the https://c2sp.org/tlog-tiles conformance checks live together.

There's precedent here, as we ran into some of these issues with static-ct.

Start by examining the hammer conformance tool and the integration checks it already runs. Map how Add requests and responses, leaf bundles, and L0 tiles are represented, then determine which consistency checks are feasible under heavy load. Done means the agreed checks are implemented and verified without allowing these references to diverge.