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_server_ratelimits.go

@@ -9,7 +9,8 @@ import (
 const JSCKServerRateLimits string = "rate_limits"
 
 // JSCServerRateLimits configures the server's rate limiters. IdleTTL is how
-// long a client's limiter is kept after its last request.
+// long a client's limiter is kept after its last request; 0 or less means
+// limiters are never evicted.
 type JSCServerRateLimits struct {
 	IdleTTL time.Duration `cfg:"idle_ttl" default:"10m"`
 }