[TEST] Restore test logging level after test is done
Core: also log byte size and doc count when we log slow merges

Closes #8855

Closes #8853
[CORE] Drop support for state written pre 0.90

Today we have several upgrade methods that can read state written
pre 0.90 or even pre 0.19. Version 2.0 should not support these state
formats. Users of these version should upgrade to a 1.x or 0.90.x version
first.
fix wording