v3.1: Streamer: allow 2000 unstaked connection slots (backport of #9289)

[mergify]

Problem

• Currently we have way more connections incoming over TPU than the size of connection table
• This results in connection churn and overall loss of efficiency due to evictions
• allow 4000 unstaked connections in TPU #8144 could not be backported, thus redoing it
• Not bumping all the way to 4000, safer change to 2000 only due to backport

Currently operators may customize this setting via hidden CLI argument --tpu-max-unstaked-connections, though most choose not to. If this change is not desired, it may be reverted at runtime by supplying desired number via CLI.

Summary of Changes

This is on top of #9288

• Bump default max unstaked connections from 500 to 2000 to allow for more unstaked senders & reduce connection table churn

Testing on a staked mainnet node, shows that 2000 connection entries in the table are sufficient to accommodate current mainnet demand during leader slots (2 leader slots captured in the plot below):

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This is an automatic backport of pull request #9289 done by [Mergify](https://mergify.com).

[t-nelson]

test breakage looks real?

[alexpyattaev]

Tests red since this lands after another PR (#9300)

[t-nelson]

ah that one is clear to merge. get it in and rebase here

[t-nelson]

@Mergifyio rebase

[mergify]

rebase

✅ Branch has been successfully rebased

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.2%. Comparing base (a1ff8b7) to head (daecdb0).

Additional details and impacted files

@@           Coverage Diff           @@
##             v3.1    #9301   +/-   ##
=======================================
  Coverage    83.2%    83.2%           
=======================================
  Files         865      865           
  Lines      375623   375623           
=======================================
+ Hits       312674   312694   +20     
+ Misses      62949    62929   -20     

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