add weak tombstone tracking and reclaimable value metrics

[zaidoon1]

to address #101

[zaidoon1]

@marvin-j97 want me to address the merge conflicts or should i hold off until you are done working on other things that may introduce more merge conflicts

[marvin-j97]

I don' think what I'm working on will meaningfully conflict with this PR.
Though I added a QoL macro in segment/meta to easier parse u64 metadata.

[zaidoon1]

@marvin-j97 did you have other tickets part of 3.0 that you need help with? The rest all seem to be assigned to you

[marvin-j97]

@marvin-j97 did you have other tickets part of 3.0 that you need help with? The rest all seem to be assigned to you

I think the only issues that can be really worked on without fear of conflicts are in fjall itself: fjall-rs/fjall#191 and fjall-rs/fjall#110 and fjall-rs/fjall#188.

[marvin-j97]

#165 is another task that needs attention - I think it's a bit more involved to read into the existing code, because the block decoding is a bit more complex, but it would be nice to have in 3.0