fix race condition in the coverage distribution actor

8ea9b2795d93f2d92d6b29c910a2e517af1f6381  output/coverage_distribution.txt-canonical

real    0m23.005s
real    0m22.824s
real    0m22.993s
real    0m21.031s
real    0m22.721s
real    0m22.884s
real    0m22.661s
real    0m22.790s
real    0m22.401s
real    0m22.855s
real    0m22.460s
real    0m21.960s
real    0m21.793s
real    0m22.841s
real    0m23.693s

Link: https://github.com/GeneAssembly/biosal/issues/481
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>