Break up AccountsIndex lock (bp #12787)

[mergify]

This is an automated backport of pull request #12787 done by Mergify

Cherry-pick of e6b821c has failed:

On branch mergify/bp/v1.4/pr-12787
Your branch is up to date with 'origin/v1.4'.

You are currently cherry-picking commit e6b821c39.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:

	modified:   Cargo.lock
	modified:   programs/bpf/Cargo.lock
	modified:   runtime/Cargo.toml
	modified:   runtime/benches/accounts.rs
	modified:   runtime/benches/accounts_index.rs
	modified:   runtime/src/accounts_index.rs
	modified:   runtime/src/bank.rs

Unmerged paths:
  (use "git add <file>..." to mark resolution)

	both modified:   runtime/src/accounts.rs
	both modified:   runtime/src/accounts_db.rs

To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

Mergify commands and options

More conditions and actions can be found in the documentation.

You can also trigger Mergify actions by commenting on this pull request:

• @Mergifyio refresh will re-evaluate the rules
• @Mergifyio rebase will rebase this PR on its base branch
• @Mergifyio update will merge the base branch into this PR
• @Mergifyio backport <destination> will backport this PR on <destination> branch

Additionally, on Mergify dashboard you can:

• look at your merge queues
• generate the Mergify configuration with the config editor.

Finally, you can contact us on https://mergify.io/

[mergify]

automerge label removed due to a CI failure

[codecov]

Codecov Report

Merging #13240 into v1.4 will increase coverage by 0.0%.
The diff coverage is 99.5%.

@@           Coverage Diff           @@
##            v1.4   #13240    +/-   ##
=======================================
  Coverage   82.1%    82.1%            
=======================================
  Files        376      376            
  Lines      86963    87097   +134     
=======================================
+ Hits       71469    71582   +113     
- Misses     15494    15515    +21