[Compatibility] Adding EXPIRETIME and EXPIRETIME command

[Vijay-Nirmal]

Adding EXPIRETIME and EXPIRETIME command to garnet

• Add EXPIRETIME and EXPIRETIME command
• Add Test cases
• Add documentation

Open Question:

1. For Object type, Should I create a single GarnetObjectType called Expiration instead of EXPIRETIME and PEXPIRETIME? The advantage will be that it will be a single generic implementation that can be reused for others in the future if required. The disadvantage is that there will be a slight performance penalty but no additional memory needed since values of Expiration, EXPIRETIME and PEXPIRETIME are long type, so we can do in-place override the value after recomputing the required value.