libsnapshot: Improve inspect_cow output.

Clean up the formatting and unit display, and add a line displaying how
long it took to parse the COW.

Example of new output:

    Version: 2.0
    Header size: 38
    Footer size: 84
    Block size: 4096
    Merge ops: 0
    Readahead buffer: 2097152 bytes
    Footer: ops usage: 1005680 bytes
    Footer: op count: 50284
    Parse time: 6.77921ms
    Data ops: 50015
    Replace ops: 50015
    Zero ops: 0
    Copy ops: 0
    Xor ops: 0

Bug: 280529365
Test: inspect_cow /dev/block/mapper/product_b-cow
Change-Id: Ie8ac02adc004289cc3a08ef44aa3048280a2407f