Refactor benchmark runner to reduce memory overhead

[beckernick]

The current benchmark runner script adds additional memory overhead in the client and worker processes relative to running an individual query. These patterns are consistent across many runs.

On a DGX-2, while running Q02 in the runner, the client process on GPU 0 uses about 900 MB of memory. On the workers, the "baseline" memory pool is 28543 MB.

|=============================================================================|
|    0     41449      C   .../envs/rapids-gpubdb-20210331/bin/python 28543MiB |
|    0     43486      C   python                                       891MiB |
|    1     41453      C   .../envs/rapids-gpubdb-20210331/bin/python 32001MiB |
|    2     41456      C   .../envs/rapids-gpubdb-20210331/bin/python 28543MiB |
...

Running the same query as a standalone script, the client process uses about 630 MB of memory. On the workers, the "baseline" memory pool is 28289 MB.

|=============================================================================|
|    0     44016      C   .../envs/rapids-gpubdb-20210331/bin/python 28289MiB |
|    0     46114      C   python                                       633MiB |
|    1     44019      C   .../envs/rapids-gpubdb-20210331/bin/python 28289MiB |
|    2     44023      C   .../envs/rapids-gpubdb-20210331/bin/python 28289MiB |