tryObjectEncoding(): use shared objects with maxmemory and non-LRU policy.

In order to make sure every object has its own private LRU counter, when
maxmemory is enabled tryObjectEncoding() does not use the pool of shared
integers. However when the policy is not LRU-based, it does not make
sense to do so, and it is much better to save memory using shared
integers.
PING: backward compatible error for wrong number of args.
Test: small integer sharing depends on maxmemory policy.
Test: Pub/Sub PING.