Graceful shutdown with the `serve` command

Out of curiosity is it easy enough to have a graceful shutdown here where ctrl-c sets a flag "please exit ASAP"? Otherwise this looks like it cancel all pending work if it's happening during ctrl-c (in which case this probably isn't much better than avoiding installing a ctrl-c in the first place)

_Originally posted by @alexcrichton in https://github.com/bytecodealliance/wasmtime/pull/7091#discussion_r1340538192_