IMPALA-491: Fixed worker process failure test and clients-in-use metric accounting.

Fixing the worker process failure test revealed a bug in our clients-in-use metrics
accounting in the client cache in the presence of node failures.
The pathological sequence causing leading to an incorrect metric was:
1. ClientConnection c'tor checks out an existing client and updates metrics
2. Existing client connection has broken pipe (or some other TException)
3. Recover by calling ClientConnection::Reopen()
4. Reopen() internally tries to create a new client connection, but the connection
   fails because the remote side is dead.
5. ClientConnection::client_ is set to NULL and therefore its d'tor
   does not properly decrement the clients-in-use for the client
   checked out in step 1.

Change-Id: I911210b23c4a024a4a8a84365a3d12268767b031
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/3804
Reviewed-by: Alex Behm <alex.behm@cloudera.com>
Tested-by: jenkins