Made each source start from its newest stored release, with --since as an override

Without --since, fetcher service and job-search serve start each source
from the newest release stored for it (the new store.LatestReleaseDate,
by Source), so a restart carries on where it left off instead of fetching
the last day again; with nothing stored, the client's default still
applies. --since overrides it for every source. The Find a Tender source's
name is now a constant, as the Source its opportunities are stored with.

The README's guide to adding a source is brought up to date with the
current worker, start and config type names.

internal/store/queries.go

@@ -23,6 +23,16 @@ package store
 const selectOpportunityLatestReleaseDate = `
 SELECT latest_release_date FROM opportunities WHERE uuid = ?`
 
+// selectSourceLatestReleaseDate reads the newest latest_release_date of one
+// source's opportunities. ORDER BY rather than MAX keeps the column's DATETIME
+// type, so the driver reads it as a time; it walks the latest_release_date
+// index.
+const selectSourceLatestReleaseDate = `
+SELECT latest_release_date FROM opportunities
+WHERE source = ?
+ORDER BY latest_release_date DESC
+LIMIT 1`
+
 // upsertOpportunity inserts or updates a opportunity. Named parameters
 // (types.Opportunity).
 //