sql: remove epoch info query cache

[ivan4th]

Motivation

The epoch info query cache mechanism contains a hard-to-fix deadlock, possibly due to a recursive RLock() on an RWMutex. Moreover, due to an oversight, in the default fetcher mode (streaming) epoch query cache is not actually being used, while the cached slice update calls which are causing deadlock are still being made upon adding each new ATX.
With syncv2, epoch info cache will no longer be needed at all, and with current network size, it isn't very useful either.

Description

This change removes epoch query cache, keeping blob query cache for now.

Test Plan

Make sure tests pass, including systests.

[codecov]

Codecov Report

Attention: Patch coverage is 70.83333% with 7 lines in your changes missing coverage. Please review.

Project coverage is 79.7%. Comparing base (b905784) to head (1a6d17c).

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
fetch/handler.go	56.2%	6 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #6575     +/-   ##
=========================================
- Coverage     79.7%   79.7%   -0.1%     
=========================================
  Files          355     355             
  Lines        47168   47097     -71     
=========================================
- Hits         37620   37561     -59     
+ Misses        7404    7394     -10     
+ Partials      2144    2142      -2     

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