Avoid races when creating night's directories.

The write of the first exposure on 2020-04-30 failed. Specifically, the creation of the /data/pfs/2020-04-30 directory failed for the b1 ccdActor because the r1 ccdActor had already created it.

Do the creates atomically or inside a try...except block.

I believe that the window for the bug is pretty narrow, but to avoid the problem for the rest of the pre-switchover SM1 acquisition, take the first exposure of the calendar day (after midnight UTC) with one camera.

I will not fix this for the existing "LAM" versions. fmadecncaplararnaud.lefur