Reduce number of Kudu clients that get created

Creating Kudu clients is very expensive as each will fetch metadata from the Kudu master. We can reduce the load on the Kudu master by reusing Kudu clients when possible. To start, we can use a single client for the entire BE and another for the entire FE.

This is dependent on a metadata invalidation improvement from Kudu (https://gerrit.cloudera.org/#/c/6719/)