scanner: batch cert last_seen updates to reduce DB lock contention

[g-k]

batch cert last_seen updates to reduce DB lock contention

refs: https://gist.github.com/g-k/7d2e8835399018a51a0407fb4337d75a and prior poking with @Micheletto

Checklist

• [x] Run make, gofmt and golint your code, and run a test scan on your local machine before submitting for review.

• [x] make

• [x] gofmt -d database/certificate.go tlsobs-scanner/analyser.go

• [x] golint

NB: gofmt and golint have a pile of changes and warnings

• [x] confirm that inserting code does not depend on last seen timestamps being updated as the scan certmap is analysed; as far as I can tell the last_seen column is just used for statistics (e.g. certs and new certs seen in the past 24h)
• [x] db.GetCertIDBySHA256Fingerprint just uses the sha256_fingerprint column
• [x] db.InsertTrustToDB,db.GetCurrentTrustIDanddb.UpdateTrustjust use theis_currentcolumn in thetrust` table
• [x] db.InsertCertificate adds a row to the certificates table without looking at existing last_seen values

I haven't run perf tests, but should be able to verify the changes in stage.

r? @ajvb

[g-k]

:tada: looks liked that worked:

• query out of the top 20 queries by DB load

• calls down and avg exec time is down to 1.38 ms (from 374.4 ms in https://gist.github.com/g-k/7d2e8835399018a51a0407fb4337d75a)

 Avg execution time
    1.38 ms

Times called
    543

Avg rows returned
    2.904

• highest p99 ~15ms from ~15s

• IO and lock wait are flat