http: use sync.Map for request-scoped vars

[mohammed90]

I received a report of a concurrent map read and map write failure at this line

caddy/modules/caddyhttp/marshalers.go

Lines 43 to 45 in ffb6ab0

	if ip, ok := GetVar(r.Context(), ClientIPVarKey).(string); ok {
	enc.AddString("client_ip", ip)
	}

The only conceivable reason for it is due to how the cache handler creates data race between cache and upstream on the same http.Request object. I cannot think of any other scenario. Whether it's truly the culprit or not, we can resolve it for all cases by using sync.Map. I contemplated creating our own opaque map struct with a mutex, but the one of use cases listed in sync.Map documentation seems to apply to our scenario:

The Map type is optimized for two common use cases: (1) when the entry for a given key is only ever written once but read many times, as in caches that only grow, or (2) when multiple goroutines read, write, and overwrite entries for disjoint sets of keys. In these two cases, use of a Map may significantly reduce lock contention compared to a Go map paired with a separate Mutex or RWMutex.

Namely, the entry for a given key is written once and read many times. I searched the code base for SetVar and GetVar. I don't see cases of overwrite, but there are a couple multiple GetVar for the same key.

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