This PR eliminates unnecessary genesis ledger rehashing when bootstrapping from
genesis, speeding up that process by about 1.5 minutes directly, depending on
hardware.

This PR also fixes a related bug: the initial best tip network would formerly
always fail on mainnet when bootstrapping from genesis, due to the daemon
becoming unresponsive while rehashing the genesis ledger. This would delay
startup by an additional number of minutes, cause the daemon to report itself as
synced while its best tip was still at genesis until the next best tip query
succeeded, and cause confusing behaviour in rosetta. This initial query should
now only fail under very specific poor network conditions.
