Prevent race condition in vocabulary cache update

[yethee]

Fix #34

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
❌ -1.21% (target: -1.00%)	✅ 36.36%

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (a79bf19)	279	201	72.04%
Head commit (e7ab188)	288 (+9)	204 (+3)	70.83% (-1.21%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#35)	11	4	36.36%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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