mappingstate: add distinctive dirty state

[alram]

Make a distinction when an operation is possible but nobackfill reservation is available vs no operation possible

And another commit to update README

[baergj]

Structure looks great - just a few tweaks needed.

[alram]

Thanks Josh, I'll address all the comments tomorrow!

[baergj]

Addresses another aspect of #14.

[alram]

updated. Quick tests:

# ./pgremapper undo-upmaps 4 --target
change possible but no backfill reservation available, try later.
# ./pgremapper undo-upmaps 0 --target
The following changes would be made to the upmap exception table:
pg 11.104: [0->3]
pg 11.107: [0->1]
pg 11.10b: [0->2]
No changes made - use --yes to apply changes.
# ./pgremapper undo-upmaps 0 4 --target
The following changes would be made to the upmap exception table:
pg 11.104: [0->3]
pg 11.107: [0->1]
pg 11.10b: [0->2]
No changes made - use --yes to apply changes.
# ./pgremapper undo-upmaps 4 0 --target
The following changes would be made to the upmap exception table:
pg 11.104: [0->3]
pg 11.107: [0->1]
pg 11.10b: [0->2]
No changes made - use --yes to apply changes.

[baergj]

LGTM!