EXPIREAT implementation

[keshavchand]

ExpireAt is same as expire (even code is pretty much copy paste), with the only difference being, it takes timestamp instead of seconds till expiry. It can be used to implement expiry in AOF, as

SET t1 100
EXPIRE t1 10

can be written as

SET t1 100
EXPIREAT t1 $t1_expiry

where $t1_expiry is evaluated during writing time to be CurrentUnixTimeInSeconds() + 10

[yashs360]

@keshavchand Thanks for the contribution. The code base has moved forward lot. Please rebase and re-submit if you are still keen to fix this. We will be cleaning up old PR's if they are inactive.