time: racy Timer access should either work or throw, not panic [1.14 backport]

[gopherbot]

@ianlancetaylor requested issue #37400 to be considered for backport to the next 1.14 minor release.

@gopherbot Please open a backport to 1.14.

[gopherbot]

Change https://golang.org/cl/221298 mentions this issue: [release-branch.go1.14] runtime: don't panic on racy use of timers

[dmitshur]

Approving because it is a serious issue without a workaround.

[gopherbot]

Closed by merging 3293174 to release-branch.go1.14.