PB connection used in verify_counter_capbilities can be closed during upgrade

The PB connection to the `legacy` node can close if the upgrade takes too long and cause the test to fail with an error similar to this:

```
================ verify_counter_capability failure stack trace =====================
{{assertEqual_failed,[{module,verify_counter_capability},
                      {line,70},
                      {expression,"riakc_pb_socket : counter_incr ( PrevPB , ? BUCKET , ? KEY , 1 )"},
                      {expected,ok},
                      {value,{error,disconnected}}]},
 [{verify_counter_capability,'-confirm/0-fun-14-',2,
                             [{file,"tests/verify_counter_capability.erl"},
                              {line,70}]},
  {verify_counter_capability,confirm,0,
                             [{file,"tests/verify_counter_capability.erl"},
                              {line,70}]}]}
====================================================================================

09:38:09.576 [notice] verify_counter_capability Test Run Complete

Test Results:
verify_counter_capability-bitcask: fail
---------------------------------------------
1 Tests Failed
0 Tests Passed
That's 0.0% for those keeping score
```

The solution is to create a new PB connection to use after the node has been upgraded and restarted.