feat: restore cache mutex

[jeffwoooo]

This restores concurrent cache mutex and fixes request freezing on ServeHTTP panic.

If ServeHTTP panics without this patch,

following codes which signal to channel won't be executed
so every requests who wait for channel will freeze

to signal regardless result of ServeHTTP,

capture request context's done event
close channel without sending value if serving was not successful

I was able to test with

force panic in any request like 'health'.
make several requests concurrently (curl ... & curl ... & curl ... & curl ... & wait)
freezing has gone!