go/store/nbs: Add info level logging for a store conjoin.

[reltuk]

No description provided.

[reltuk]

Operationally, conjoins can be expensive. With auto GC, it's more likely that we see these in old gen going forward.

We are working on making conjoins less impactful on a server's resource utilization and working on making them less async, so their lock overhead is much lower, but operationally it is still useful to log when they are occuring.

[coffeegoddd]

@reltuk DOLT

comparing_percentages
100.000000 to 100.000000

version	result	total
88245a5	ok	5937457

version	total_tests
88245a5	5937457

correctness_percentage
100.0

[zachmu]

LGTM