Support "commitments" in Home and Replica

[asaj]

This issue is to replace the existing "update" model for synchronizing Home and Replica contracts.

The Home contract will no longer keep a queue of merkle roots. Instead, upon calls to an external function snapshot, it will store a mapping(root -> leaf index).

The updater will wait the appropriate confirmation time and then sign a commitment to (root, leaf index). Replicas will accept this commitment iff the leaf index is greater than the greatest leaf index that they've already committed.

Updaters are slashable iff they've signed a commitment to a snapshot that is not present in the mapping on Home.