Add StatusCache::root_slot_deltas() and use it

[brooksprumo]

Problem

All uses of StatusCacheRc::slot_deltas() (1) only get the roots from the status cache, and (2) unnecessarily sort. These are performance loses.

Summary of Changes

1. Add StatusCache::root_slot_deltas() which gets the slot deltas from its roots
2. Remove StatusCacheRc::slot_deltas() and call StatusCache::root_slot_deltas() instead
3. Add benchmarks to ensure this is actually a speedup

1. test bench_status_cache_root_slot_deltas               ... bench:       8,366 ns/iter (+/- 592)
2. test bench_status_cache_slot_deltas                    ... bench:       9,200 ns/iter (+/- 612)
3. test bench_status_cache_slot_deltas_get_roots_sorted   ... bench:      16,086 ns/iter (+/- 2,978)
4. test bench_status_cache_slot_deltas_get_roots_unsorted ... bench:      10,692 ns/iter (+/- 809)

In (2), that is the current call to StatusCache::slot_deltas(). However, no code just calls (2); they all call StatusCache::roots() first. So (3) and (4) show the true results for the current way we get the slot deltas (really just (3)). And (1) is the new way.

The existing code performed a sort on the roots passed into slot_deltas(), but this is unnecessary. The vector returned is never indexed; in fact, it's only used for the snapshot, and its deserialization puts the vector back into a HashMap.

Getting the slot deltas is called during bank_forks::set_root(), so anything to speed that path up is useful.

Note: benches 3 and 4 are not part of the final PR. They are in the first commit, should you like to look at them and/or run them.

[codecov]

Codecov Report

Merging #26170 (033fe6b) into master (1165a7f) will increase coverage by 0.0%.
The diff coverage is 90.6%.

@@           Coverage Diff           @@
##           master   #26170   +/-   ##
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  Coverage    81.9%    81.9%           
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  Files         631      631           
  Lines      174252   174247    -5     
=======================================
+ Hits       142728   142767   +39     
+ Misses      31524    31480   -44     

[HaoranYi]

lgtm.

[ryoqun]

@brooksprumo hey, coming from #26666, i have some comments here. :)