common, core, light: add block age into info logs

[karalabe]

While catching up with the chain (you turned off your machine for a few days), it's generally annoying that you don't know where in the sync you are progress wise. Checking against a block explorer is of course an option, but it would be nice if the logs helped you a bit out.

This PR adds an age field to all the import logs (receipt, block, header) and also to the "loaded most recent" logs. This hopefully provides a bit more context as to how long a catch-up might take:

Startup logs:

INFO [09-20|11:39:23.581] Loaded most recent local header          number=3002486 hash=0a0b16…555511 td=5587832 age=3d2h38m
INFO [09-20|11:39:23.581] Loaded most recent local full block      number=3002486 hash=0a0b16…555511 td=5587832 age=3d2h38m
INFO [09-20|11:39:23.581] Loaded most recent local fast block      number=3002486 hash=0a0b16…555511 td=5587832 age=3d2h38m

Import logs:

INFO [09-20|12:11:26.785] Imported new block headers               count=384 elapsed=328.229ms number=4074232 hash=633555…f35f7f age=10h15m48s
INFO [09-20|12:11:26.976] Imported new block headers               count=192 elapsed=174.491ms number=4074424 hash=2aa83c…a08d66 age=9h31m36s
INFO [09-20|12:11:27.465] Imported new block headers               count=576 elapsed=469.789ms number=4075000 hash=a32347…47afd9 age=7h25m36s
INFO [09-20|12:11:27.644] Imported new block headers               count=192 elapsed=162.887ms number=4075192 hash=274c76…d5b962 age=6h47m5s
INFO [09-20|12:11:28.334] Imported new block headers               count=768 elapsed=670.033ms number=4075960 hash=9397fe…8dd879 age=3h54m55s
INFO [09-20|12:11:28.647] Imported new block headers               count=384 elapsed=290.009ms number=4076344 hash=b69eea…c5a0e2 age=2h31m51s
INFO [09-20|12:11:29.113] Imported new block headers               count=576 elapsed=447.128ms number=4076920 hash=c64a30…7894ab age=29m2s
INFO [09-20|12:11:29.243] Imported new block headers               count=127 elapsed=110.961ms number=4077047 hash=ea1c6b…eb6b18 age=35s
INFO [09-20|12:11:50.367] Imported new block headers               count=2   elapsed=5.152ms   number=4077049 hash=d59587…a766ac age=9s
INFO [09-20|12:12:02.047] Imported new block headers               count=1   elapsed=11.174ms  number=4077050 hash=b1db79…07c590 age=13s

Note, this PR drops the mgas log field from imports. It's a cute statistic, but it's not really useful. Blocks are generally full, so there's not much surprise in mgas. Also, the mgas is just the elapsed * mgasps that we sill log so you can calculate it if you really must.

[holiman]

Although I think mgas is good to have. I don't think it's quite replaceable: I doubt you can losslessly calculate it from the pretty-printed mgasps and elapsed.

[karalabe]

But is it useful? If I import blocks 1-by-1 (i.e. I'm in sync), it will just hover around 8M (or whatever mainnet is pushed to). If I'm importing a lot of blocks (random number), then the mgas loses it's value, because it's just some average across a few hundred blocks. I'm genuinely curious, I can put it back if it's useful.

[ghost]

ACK for this feature

[karalabe]

Pushed a tiny fixup:

• Restore the mgas field.
• Raise the age display threshold from 2 secs to 1 minute (avoid propagation confusion)
• Add the age to the light client CHT restoration log.