fix: ignore messages that come in after close

[callmehiphop]

Fixes #483

• Tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

[codecov]

Codecov Report

Merging #485 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #485   +/-   ##
=======================================
  Coverage   95.11%   95.11%           
=======================================
  Files          15       15           
  Lines         982      982           
  Branches       73       73           
=======================================
  Hits          934      934           
  Misses         43       43           
  Partials        5        5

Impacted Files	Coverage Δ
src/lease-manager.ts	100% <100%> (ø)	⬆️
src/subscriber.ts	96.84% <100%> (ø)	⬆️

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