Keep graph traversal indecies (tx_out_end / tx_in_end) in memory during build #71

Every non-coinbase input becomes 2 pread(2) syscalls of 28 bytes into a file we just appended to. This is the ~same order of ops as appends.
Fix: during parse_blocks, keep Vec shadows of tx_out_end (and tx_in_end if needed) indexed by TxId. Read them from memory. Flush happens as a side effect of txptr_index.append(...). . Periodically will need to flush regardless