Prepare to resurrect TS_RESP_CTX_set_time_cb()

Prepare to resurrect TS_RESP_CTX_set_time_cb()

This was removed shortly after the fork since TS is not 2038-ready
and since there were no consumers of this API. Now there are consumers
and they add it themselves if it's missing from libcrypto. This will no
longer be possible with opaque TS structs, so begrudgingly add it back.

ok jsing kn