Introduce a MemcacheCommand class to encapsulate some of the common
parameters and functionality associated with the MemcacheRing methods.
No behavioural changes.
Change-Id: I2a84d5b74d74967628973ad5d67749b787ba7590
Related-Change: I4d4f20b92b85255ac8bf66f2c830e691e64bbe47
Related-Change: I07491bb4ebc3baa13cf09f64a04a61011d561409