[Hammer] Monitor leaf integration

[phbnf]

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.