Fix pre-check of blockstore slots during load_bank_forks

[apfitzge]

Problem

Check I added in #25329 was too restrictive. It's possible we having missing blocks in the range, but can still generate a snapshot.

Summary of Changes

Update the blockstore slots check to iterate from the starting slot -> halt_slot using slot_meta.next_slots.

Fixes #

[codecov]

Codecov Report

Merging #25632 (e2e3ccc) into master (02b26dd) will increase coverage by 0.0%.
The diff coverage is n/a.

❗ Current head e2e3ccc differs from pull request most recent head 91120cb. Consider uploading reports for the commit 91120cb to get more accurate results

@@           Coverage Diff           @@
##           master   #25632   +/-   ##
=======================================
  Coverage    82.1%    82.1%           
=======================================
  Files         621      621           
  Lines      170297   170297           
=======================================
+ Hits       139917   139955   +38     
+ Misses      30380    30342   -38     

[apfitzge]

Measured the time this check takes on the case where I ran into in devnet:

starting_slot=136943427
halt_slot=136944100
difference=673
slots_connected took 4ms

so approximately 6us/slot, though probably some noise there since it's a small sample size. Checking an entire epoch's worth of slots that'd take roughly 2.5s base on this measurement.

[steviez]

LGTM