set loadtime during initial transfer of a secondary zone

Evan Hunt requested to merge 3983-secondary-loadtime into main

when transferring in a non-inline-signing secondary for the first time, we previously never set the value of zone->loadtime, so it remained zero. this caused a test failure in the statschannel system test, and that test case was temporarily disabled. the value is now set correctly and the test case has been reinstated.

Closes #3983 (closed)

Edited by Evan Hunt

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