Add a lock to the rate-limit dict

[mayhem]

As per discussion in matrix, we suspect that the dict is being overwritten by another thread.

[github-actions]

Test Results

  4 files  ±0    4 suites  ±0   4s ⏱️ ±0s
 50 tests ±0   50 ✅ ±0  0 💤 ±0  0 ❌ ±0 
200 runs  ±0  200 ✅ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit 61dcbf2. ± Comparison against base commit e4d6630.

[github-actions]

Package	Line Rate	Complexity	Health
.	33%	0	❌
content_resolver	0%	0	❌
content_resolver.formats	6%	0	❌
content_resolver.model	0%	0	❌
external	39%	0	❌
listenbrainz	46%	0	❌
musicbrainz	72%	0	➖
patches	27%	0	❌
patches.lb_radio_classes	0%	0	❌
tools	20%	0	❌
.	33%	0	❌
content_resolver	0%	0	❌
content_resolver.formats	6%	0	❌
content_resolver.model	0%	0	❌
external	39%	0	❌
listenbrainz	46%	0	❌
musicbrainz	72%	0	➖
patches	27%	0	❌
patches.lb_radio_classes	0%	0	❌
tools	20%	0	❌
.	33%	0	❌
content_resolver	0%	0	❌
content_resolver.formats	6%	0	❌
content_resolver.model	0%	0	❌
external	39%	0	❌
listenbrainz	46%	0	❌
musicbrainz	72%	0	➖
patches	27%	0	❌
patches.lb_radio_classes	0%	0	❌
tools	20%	0	❌
.	33%	0	❌
content_resolver	0%	0	❌
content_resolver.formats	6%	0	❌
content_resolver.model	0%	0	❌
external	39%	0	❌
listenbrainz	46%	0	❌
musicbrainz	72%	0	➖
patches	27%	0	❌
patches.lb_radio_classes	0%	0	❌
tools	20%	0	❌
Summary	21% (4295 / 20170)	0	❌