Fix Ledger block hashing and caching.

Fix Ledger block hashing and caching.
Ledger app requires open blocks to have zeroed frontier bytes instead of account public key, diverging from spec, so zero out "parent" block like Ledger expects if public key is the frontier.
Ensure block data is correct length before hashing.
Merge shared functionality of signing blocks and nonces, and document bug with Ledger nano app that is preventing nonces from being signed.
Skip online tests for now.