git_block_ids crashed when forkdb had wrong last_irreversible_block

Renamed forkdb's last_irreversible_block to get_oldest_block_num_unlocked, because while it is always the oldest block in the forkdb, during early block processing is it not necessarily an irreversible block.

Also needed to change implementation of get_block_ids so that it gets this oldest block from the forkdb and not from the block_log, and this is what fixed the crash. The problem case occurred when this oldest block was not irreversible (and hence not in the block_log) but the function still tried to get it from the block_log.