Minor Description
A user on the mailing list recently pointed out that if it's not specified explicitly as a Builder option, ConcurrentUpdateSolrClient will default to using a queueSize of 0. This value gets passed in to the underlying queue data structure which throws an IllegalArgumentException, with an error message that isn't obvious to those unfamiliar with the internals.
We should provide a better default than the uninitialized-variable default of 0. Almost all occurrences in the code fall between 5 and 10, so a queueSize in that range should be uncontroversial.