store first load time in DB, not rely on blockchain timestamps

[synfinatic]

Basically, you can have big gaps where there aren't any beacons/challenges/witnesses because a node is offline or doesn't exist and I should ideally store a marker or something to indicate the first time so the "first database record" is the start of the graph range

Here is the output after a 2nd run:

./dist/helium-analysis graph passive-umber-rat -L debug
DEBU[0000] Graph range: 2021-03-09 00:00:00 UTC => 2021-04-08 03:26:30 UTC
DEBU[0000] Database challenges: 2021-03-12 17:13:28 UTC => 2021-04-08 02:17:51 UTC
INFO[0000] First database record is after 2021-03-09 06:00:00 UTC
DEBU[0000] Loading challenges until: 2021-03-08 18:00:00 UTC
INFO[0075] Loaded 100 challenges
INFO[0075] Last challenge time: 2021-04-06 00:07:04 PDT
INFO[0151] Loaded 200 challenges
INFO[0151] Last challenge time: 2021-04-04 16:28:50 PDT
INFO[0228] Loaded 300 challenges
INFO[0228] Last challenge time: 2021-04-03 08:25:09 PDT
INFO[0305] Loaded 400 challenges
INFO[0305] Last challenge time: 2021-04-01 21:25:53 PDT
INFO[0381] Loaded 500 challenges
INFO[0381] Last challenge time: 2021-03-13 09:57:28 PST
DEBU[0388] found 505 challenges for 112p9N1GsXhNK2hbZJ4K8MoqVbH1DJ3r4NTFrHgMU6sFn1h8bU46
INFO[0388] Loaded 2 new challenges into database
INFO[0388] Created passive-umber-rat/beacon-totals.png
INFO[0388] Created passive-umber-rat/witness-distance.png