[TieredStorage] Have the reader until TieredStorage lazy initialized

[yhchiang-sol]

Problem

Currently, right after TieredStorage::write_accounts() is called, it
immediately mmap the file in case it is the hot storage. It might
be more suitable to delay the mmap until the file is actually read.

Summary of Changes

Have the reader under the TieredStorage to be lazy initialized.

Test Plan

Existing unit-tests.

Dependency

#35049

[codecov]

Codecov Report

Attention: Patch coverage is 93.65079% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 81.6%. Comparing base (dd30175) to head (517ca3f).
Report is 131 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #35063   +/-   ##
=======================================
  Coverage    81.6%    81.6%           
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  Files         830      830           
  Lines      224946   224966   +20     
=======================================
+ Hits       183674   183709   +35     
+ Misses      41272    41257   -15     

[brooksprumo]

Also, bigger question, is wirte_accounts() here thread-safe? What happens if different threads call this method on the same instance?

[brooksprumo]

Seems like overkill here. Wdyt about using an AtomicBool instead?

[yhchiang-sol]

Sounds good. I am also thinking about using u64 (i.e., AtomicU64) that represents file size instead so that we can have a faster file_size() as well as checking whether is read-only.

[yhchiang-sol]

Making this PR draft again as the thread-safety part has been move to a dedicate PR #35143

[willhickey]

This repository is no longer in use. Please re-open this pull request in the agave repo: https://github.com/anza-xyz/agave