disk index: set_anticipated_count to optimally grow disk buckets at startup

[jeffwashington]

Problem

See #30711
At startup, we know how many inserts are soon coming.

This appears to cut ~15s off startup time and should greatly reduce disk i/o during startup.
This is a candidate for 1.14 if we want to enable disk index.

Summary of Changes

Tell the disk index code how many inserts are coming so that the resizing algorithm doesn't go through unnecessary incremental regrows.
This becomes very important when we stop doubling the index size each time we grow the index bucket.

Fixes #

[codecov]

Codecov Report

Merging #31033 (f551092) into master (f289a07) will increase coverage by 0.0%.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##           master   #31033   +/-   ##
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  Coverage    81.5%    81.5%           
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  Files         728      728           
  Lines      205553   205567   +14     
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+ Hits       167649   167661   +12     
- Misses      37904    37906    +2