[aggregator] make sure worker pool is at least 1 (#1881)
[aggregator] make sure worker pool is at least 1 (#1881)

It is possible to configure the aggregator so that the worker pool size ends up being 0. In that case, the aggregator does not perform any work, but also do not return any error messages making it very hard to understand why nothing is happening
m3db/m3db,m3db/m3db,m3db/m3db,m3db/m3db