MemcachedCache#get doesn't ensure it returns the results for the key its asked

[kgyrtkirk]

this might be a theoretical level issue; but:

• the get method maps the key to a hash and looks that up
• the hash uses sha1 (which should provide enough spread)

however it can't be ruled out to have 2 key mapping to the same hash value.

the current implementation doesn't validate that it got the cached value for that key or some other.

druid/server/src/main/java/org/apache/druid/client/cache/MemcachedCache.java

Lines 485 to 500 in 423c91f

	future = clientHolder.get().asyncGet(computeKeyHash(memcachedPrefix, key));
	}
	catch (IllegalStateException e) {
	// operation did not get queued in time (queue is full)
	errorCount.incrementAndGet();
	log.warn(e, "Unable to queue cache operation");
	return null;
	}
	try {
	byte[] bytes = (byte[]) future.get(timeout, TimeUnit.MILLISECONDS);
	if (bytes != null) {
	hitCount.incrementAndGet();
	} else {
	missCount.incrementAndGet();
	}
	return bytes == null ? null : deserializeValue(key, bytes);