Simplified usage of use arrow in ballista.

[jorgecarleitao]

Closes #446

Also simplifies many of the internal imports, removing unused imports and place them all at the top of the module / file.

[codecov-commenter]

Codecov Report

Merging #447 (6861cf8) into master (321fda4) will increase coverage by 0.00%.
The diff coverage is 62.31%.

@@           Coverage Diff           @@
##           master     #447   +/-   ##
=======================================
  Coverage   75.16%   75.16%           
=======================================
  Files         150      150           
  Lines       25144    25142    -2     
=======================================
  Hits        18899    18899           
+ Misses       6245     6243    -2     

Impacted Files	Coverage Δ
ballista/rust/client/src/columnar_batch.rs	0.00% <ø> (ø)
ballista/rust/client/src/context.rs	0.00% <ø> (ø)
ballista/rust/core/src/client.rs	0.00% <ø> (ø)
ballista/rust/core/src/datasource.rs	0.00% <ø> (ø)
ballista/rust/core/src/error.rs	0.00% <ø> (ø)
...lista/rust/core/src/execution_plans/query_stage.rs	42.85% <ø> (ø)
...ta/rust/core/src/execution_plans/shuffle_reader.rs	0.00% <ø> (ø)
...ust/core/src/execution_plans/unresolved_shuffle.rs	50.00% <ø> (ø)
ballista/rust/core/src/memory_stream.rs	0.00% <ø> (ø)
...ta/rust/core/src/serde/physical_plan/from_proto.rs	39.78% <ø> (ø)
... and 9 more

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