Validation timing fixes:
* Log whether consensus built a ledger we already had or were acquiring
* Don't trigger an acquire for a validation for a ledger we might be building
* When we finish building a ledger, try to accept that ledger
* If we cannot accept a built ledger, check held validations
* Correctly set isCurrent for untrusted validations
* Add appropriate logging

This fixes a race condition that could cause spurious and expensive
ledger fetches across the network and delayed recognition of
fully-validated ledger.
Add comments to LedgerHistory
Set version to 0.24.0-rc1