This change is the only one necessary to allow future versions of
the microdescriptor consensus to replace every 'published' date with
e.g. 2038-01-01 00:00:00; this will save 50-75% in compressed
microdescriptor diff size, which is quite significant.

This commit is a minimal change for 0.2.9; future series will
reduce the use of the 'published' date even more.

Implements part of ticket 21642; implements part of proposal 275.