Make linearizer more quiet

[hawkowl]

It's like 25% of our logs and not very useful at INFO

[codecov-io]

Codecov Report

Merging #4507 into develop will decrease coverage by <.01%.
The diff coverage is 80%.

@@             Coverage Diff             @@
##           develop    #4507      +/-   ##
===========================================
- Coverage    74.75%   74.75%   -0.01%     
===========================================
  Files          336      336              
  Lines        34156    34156              
  Branches      5554     5554              
===========================================
- Hits         25535    25534       -1     
  Misses        7046     7046              
- Partials      1575     1576       +1

[erikjohnston]

FTR, the reason why this was added was that it helped track down bugs where synapse would lock up entirely (due to one instances taking out the lock and then blocking).

I agree that they're really noisy.

[richvdh]

we might find ourselves needing to turn them back up again (or replace them with something more targeted), but lgtm

[ara4n]

tracing these (very noisy) lineariser logs was the only way that mjark & I were able to diagnose the locks which led to the federation meltdown of Dec 2016, fwiw. i subsequently have used them quite a lot to see which locks are causing deadlocks and meltdowns. i find myself echoing vdh's last comment.