plug in new ledger, from #813

[rob-solana]

accomplished:

1. move write_stage to use LedgerWriter
2. move fullnode to LedgerReader
   2a. change bin/fullnode to take a ledger directory
3. move multinode tests to allow validators to have their own ledger directories
4. add LedgerWriter to validators (in ReplicateStage)

next steps:

1. add LedgerWindow to run_window_request
2. delete dead EntryWriter code

[rob-solana]

@garious I want to know what you think of the placement of LedgerWriter in the validator's replicate stage

[garious]

Is the idea that EntryWriter will be deleted and so you're interacting with LedgerWriter directly from WriteStage?

[rob-solana]

yeah, EntryWriter is not in use any more, will add removal to the TODOs... the big change is that validators are also keeping a persistent ledger, and it's being pooped out in ReplicateStage