Changing representation of datetime from time_t to int64_t

A datetime in Etch is always 64 bit long, independent of the platform.
However the time_t datatype depends on the platform so it is not a good
choice here.

Change-Id: I78cfc6962a600a635bac74522b886a3cfd765f6a

git-svn-id: https://svn.apache.org/repos/asf/etch/trunk@1578861 13f79535-47bb-0310-9956-ffa450edef68