Config changes

- a source's interval must be greater than 0: a source is turned off with
  enabled, not with a zero interval, so JSCSourcesSource rejects an interval of
  0 or less (its validate is promoted to each source's section);
- max_message_size rejects -1: JSCServerWebSocket rejects a max_message_size
  of -1 (util.ByteSizeNoLimit), as a client could then exhaust the server's
  memory;
- every setting documents what 0 or empty means (zero/none, no limit or a
  default).

internal/config/conf_source_ftsmock.go

@@ -6,7 +6,8 @@ import "reflect"
 const JSCKSourceFTSMock string = "fts_mock"
 
 // JSCSourceFTSMock configures fetching from a Find a Tender mock
-// server (cmd/mock-fts) at APIRoot.
+// server (cmd/mock-fts) at APIRoot, which has no default and must be set when
+// the source is enabled.
 type JSCSourceFTSMock struct {
 	JSCSourcesSource
 	APIRoot string `cfg:"api_root"`