util: Convert CachedValue to an ES6 class.
util: Convert CachedValue to an ES6 class.

Signed-off-by: Anders Kaseorg <dfdb7392591db597bc41cf266a9c3bc12a2706e5@zulip.com>

kou/zulip,eeshangarg/zulip,eeshangarg/zulip,brainwane/zulip,kou/zulip,eeshangarg/zulip,zulip/zulip,hackerkid/zulip,hackerkid/zulip,andersk/zulip,rht/zulip,brainwane/zulip,andersk/zulip,showell/zulip,eeshangarg/zulip,kou/zulip,punchagan/zulip,hackerkid/zulip,andersk/zulip,punchagan/zulip,eeshangarg/zulip,zulip/zulip,hackerkid/zulip,punchagan/zulip,showell/zulip,andersk/zulip,brainwane/zulip,brainwane/zulip,zulip/zulip,eeshangarg/zulip,punchagan/zulip,rht/zulip,punchagan/zulip,hackerkid/zulip,rht/zulip,zulip/zulip,kou/zulip,zulip/zulip,hackerkid/zulip,andersk/zulip,kou/zulip,showell/zulip,zulip/zulip,andersk/zulip,punchagan/zulip,rht/zulip,hackerkid/zulip,showell/zulip,rht/zulip,kou/zulip,showell/zulip,eeshangarg/zulip,showell/zulip,punchagan/zulip,zulip/zulip,brainwane/zulip,kou/zulip,rht/zulip,showell/zulip,andersk/zulip,brainwane/zulip,rht/zulip,brainwane/zulip