These changes augment the Validations class with a LedgerTrie to better
track the history of support for validated ledgers. This improves the
selection of the preferred working ledger for consensus. The Validations
class now tracks both full and partial validations. Partial validations
are only used to determine the working ledger; full validations are
required for any quorum related function. Validators are also now
explicitly restricted to sending validations with increasing ledger
sequence number.
- Separate `Scheduler` from `BasicNetwork`.
- Add an event/collector framework for monitoring invariants and calculating statistics.
- Allow distinct network and trust connections between Peers.
- Add a simple routing strategy to support broadcasting arbitrary messages.
- Add a common directed graph (`Digraph`) class for representing network and trust topologies.
- Add a `PeerGroup` class for simpler specification of the trust and network topologies.
- Add a `LedgerOracle` class to ensure distinct ledger histories and simplify branch checking.
- Add a `Submitter` to send transactions in at fixed or random intervals to fixed or random peers.
Co-authored-by: Joseph McGee
