Keep old clients from removing message rate delay

Keep old clients from removing message rate delay

Add a guard against zero message delay in setMessageRateDelay,
following the error-checking of setMessageRateBurstSize.  This blocks
old clients from setting message rate delay to 0 seconds for newly-
created networks, improperly overriding the default of 2.2 seconds.

This affects creating a network in an old client, switching to a new
client, then enabling custom rate-limits.

The supported method for removing rate limits by setting Unlimited to
true still works.

Resolves GH-252.