pkg/coveragedb: fix schema in order to support multiple managers #14937

Job	Run time
build	11m 48s
aux	4m 53s
arch (presubmit_arch_linux)	11m 5s
dashboard	3m 29s
arch (presubmit_arch_freebsd)	6m 43s
arch (presubmit_arch_netbsd)	4m 25s
old	7m 1s
arch (presubmit_arch_openbsd)	4m 19s
race	8m 33s
gvisor	4m 18s
arch (presubmit_arch_darwin)	4m 22s
arch (presubmit_arch_windows)	3m 31s
race_dashboard	4m 18s
arch (presubmit_arch_executor)	3m 28s
	1h 22m 13s

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