Read genesis parameters within ParametersState

[sandtreader]

The GenesisBootstrapper currently sends the Conway parameters as part of its startup message, which the ParametersState holds until needed. However there are also parameter updates in (e.g.) Shelley, and it would all get a bit messy.

It would be better if the GenesisBootstrapper watched block headers (see EpochActivityCounter for example) spotted the epoch boundaries for new eras (e.g. 208 for Shelley), and sent a ParametersUpdate message at the relevant time. We could also split out the parameters update from the EnactState message sent by GovernanceState, and ParametersState would then only need to listen for that.

We also need the GenesisBootstrapper to generate 'fake' SPO registrations from the genDelegs array (see #38 )

[sandtreader]

Update - complete rethink... We should fork the genesis file reading code (build.rs) from genesis_bootstrapper into parameters_state (moving all the Conway stuff, which genesis_bootstrapper will no longer need), and ParametersState should read the files at the relevant era boundaries. The Conway stuff can be removed from the genesis-complete message.

Also, we need to implement all the parameters in the different eras - Shelley is the most important because it's needed for reward calculations (#8) and send out parameter update messages with everything set, to save the receiver (e.g. AccountsState) having to manage deltas.