merge window and ledger

[rob-solana]

Problem

A ledger and window keyed by entry_height does not accommodate forks or the skip-list form of the ledger as transmitted by rotating leaders.

Proposed Solution

Move ledger and window to rocksdb, keyed by <leader_period, index> where index is relative to the leader period. The db should persist blobs directly in order to retain the signatures of the leaders for repair responses.

This effectively merges window and ledger.

Replicate stage and repair would/could be moved to consuming the ledger directly instead of consuming deserialized vectors of Entries.

The TVU's ledger_write_stage goes away.

Deserialization from the ledger moves to replicate stage (I think).

[rob-solana]

@carllin working on this in the form of db_ledger on rocksdb

cc #1888 #1900 #1885 #1875 #1851 #1792 #1712