Renames AccountsBackgroundService's thread

[brooksprumo]

Problem

The AccountsBackgroundService thread has a non-ideal name, "solBgAccounts".

1. Since we also have the accounts-db foreground and background thread pools, there's some ambiguity between the ABS thread and the bg thread pool, named "solAcctsDbBg". It's unlikely that someone without preexisting knowledge will understand the difference.

2. There is precedent for naming the thread of services as "Svc", but ABS does not follow this convention. Here's some examples:

core/src/warm_quic_cache_service.rs
69:            .name("solWarmQuicSvc".to_string())

core/src/repair/ancestor_hashes_service.rs
254:            .name("solAncHashesSvc".to_string())

core/src/repair/repair_service.rs
453:                .name("solRepairSvc".to_string())

core/src/commitment_service.rs
80:                    .name("solAggCommitSvc".to_string())

core/src/cost_update_service.rs
34:            .name("solCostUpdtSvc".to_string())

runtime/src/prioritization_fee_cache.rs
190:                .name("solPrFeeCachSvc".to_string())

rpc/src/rpc_service.rs
810:            .name("solJsonRpcSvc".to_string())

Summary of Changes

Rename the ABS thread to "solAcctsBgSvc".

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.0%. Comparing base (54205f5) to head (f92a753).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##           master    #8140     +/-   ##
=========================================
- Coverage    83.0%    83.0%   -0.1%     
=========================================
  Files         825      825             
  Lines      361899   361899             
=========================================
- Hits       300499   300421     -78     
- Misses      61400    61478     +78     

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[brooksprumo]

@steviez - requesting your review as "thread czar"

[steviez]

requesting your review as "thread czar"

😆