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_test.go

@@ -33,6 +33,9 @@ var checks = map[string]queryCheck{
 	"selectOpportunityLatestReleaseDate": {
 		sql: selectOpportunityLatestReleaseDate,
 	},
+	"selectSourceLatestReleaseDate": {
+		sql: selectSourceLatestReleaseDate,
+	},
 	"upsertOpportunity":       {sql: upsertOpportunity, named: true},
 	"selectOpportunityByUUID": {sql: selectOpportunityByUUID},
 	"listOpportunitiesSelect": {