Only import BlazingSQL as necessary (#109)

BlazingSQL is broken: BlazingDB/blazingsql#1037

This workaround allows the repo to work when BlazingSQL isn't enabled.

tpcx_bb/xbb_tools/cluster_startup.py

@@ -22,7 +22,6 @@
 import dask
 from dask.distributed import Client
 from dask.utils import parse_bytes
-from blazingsql import BlazingContext
 
 
 def get_bsql_config_options():
@@ -127,6 +126,7 @@ def maybe_create_worker_directories(dask_worker):
 
     bc = None
     if create_blazing_context:
+        from blazingsql import BlazingContext
         bc = BlazingContext(
             dask_client=client,
             pool=os.environ.get("BLAZING_POOL", False),