Add support for millisecond precision expiration

Add support for two commands: pexpire and pexpireat

Fix behavior of expire command; while expire only takes whole seconds
for its interval, the resulting ttl still has millisecond precision.
That is, the key will expire exactly (+/- 1ms) that number of seconds
later.

Change-Id: I75b1248567a1f191e35d5dbec40fd1b981df5650
Reviewed-on: https://gerrit.causes.com/23979
Tested-by: Lann Martin <lann@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>