Record begin and end timestamps for relays

[thomash-acinq]

For fighting jamming attempts, or even just to detect one, we need to know how fast relayed HTLCs are fulfilled. We now measure this and store it in the audit database. Prevously the "IN" and "OUT" directions for the same HTLC were storing the same timestamp (corresponding to when the HTLC is fulfilled), we now use the timestamp at which we received the UpdateAddHtlc for the "IN" direction.

[codecov-commenter]

Codecov Report

Merging #2701 (1504f8c) into master (1519dd0) will decrease coverage by 0.08%.
The diff coverage is 100.00%.

❗ Current head 1504f8c differs from pull request most recent head c870277. Consider uploading reports for the commit c870277 to get more accurate results

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@@            Coverage Diff             @@
##           master    #2701      +/-   ##
==========================================
- Coverage   85.98%   85.91%   -0.08%     
==========================================
  Files         215      215              
  Lines       17707    17708       +1     
  Branches      759      758       -1     
==========================================
- Hits        15226    15213      -13     
- Misses       2481     2495      +14     

Impacted Files	Coverage Δ
...main/scala/fr/acinq/eclair/db/DbEventHandler.scala	93.50% <ø> (ø)
...in/scala/fr/acinq/eclair/channel/ChannelData.scala	100.00% <100.00%> (ø)
...c/main/scala/fr/acinq/eclair/db/pg/PgAuditDb.scala	99.68% <100.00%> (ø)
...cala/fr/acinq/eclair/db/sqlite/SqliteAuditDb.scala	99.66% <100.00%> (ø)
.../scala/fr/acinq/eclair/payment/PaymentEvents.scala	97.77% <100.00%> (+0.05%)	⬆️
.../scala/fr/acinq/eclair/payment/PaymentPacket.scala	90.82% <100.00%> (ø)
...a/fr/acinq/eclair/payment/relay/ChannelRelay.scala	95.68% <100.00%> (ø)
...cala/fr/acinq/eclair/payment/relay/NodeRelay.scala	97.87% <100.00%> (ø)
.../eclair/payment/relay/PostRestartHtlcCleaner.scala	90.17% <100.00%> (ø)

... and 9 files with indirect coverage changes

[t-bast]

This will be very useful!

[t-bast]

Looks mostly good to me, just a comment on when the metric is updated.

[t-bast]

LGTM 👍