Add support for NX, XX, GT, LT, etc. to EXPIRE and EXPIREAT commands

[JyotinderSingh]

The EXPIREAT command supports a set of options: NX -- Set expiry only when the key has no expiry XX -- Set expiry only when the key has an existing expiry GT -- Set expiry only when the new expiry is greater than current one LT -- Set expiry only when the new expiry is less than current one A non-volatile key is treated as an infinite TTL for the purpose of GT and LT. The GT, LT and NX options are mutually exclusive.

We need to add support for these options in our implementations of these commands.

[raghavbabbar]

Could you please assign this to me ? @JyotinderSingh

[JyotinderSingh]

Could you please assign this to me ? @JyotinderSingh

Assigned.