Implement Last-Modified header returned by REST endpoint correctly

As described in ISPN-3153, the Last-Modified HTTP header does not always return correct date/time of the last modification.

The goal of this JIRA is to fix this behaviour.

Explanation of the bug:
"Since immortal entries never die, we don't track creation time for them. We don't wanna be doing that either cos it takes valuable space that's might only be relevant for REST. In fact, we don't have anything that maps directly to last-modified per se. For transient entries, we track when the entry was last used (read or write) and for mortal entries, we track the creation time, which is not necessarily last modified time. Last-modified returns the expected value only when timeToLiveSeconds has been set and we're tracking creation time (mortal entry)."