Refactor Ballista executor so that FlightService delegates to an Executor struct

[andygrove]

Which issue does this PR close?

Closes #449 .

Rationale for this change

This is fixing some tech debt so that the executor doesn't have to connect to itself over Flight to run queries.

What changes are included in this PR?

Are there any user-facing changes?

No

[andygrove]

@edrevo fyi

[codecov-commenter]

Codecov Report

Merging #450 (7a969c1) into master (bdae93b) will increase coverage by 0.00%.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master     #450   +/-   ##
=======================================
  Coverage   75.16%   75.17%           
=======================================
  Files         150      151    +1     
  Lines       25142    25140    -2     
=======================================
  Hits        18899    18899           
+ Misses       6243     6241    -2     

Impacted Files	Coverage Δ
ballista/rust/executor/src/execution_loop.rs	0.00% <0.00%> (ø)
ballista/rust/executor/src/executor.rs	0.00% <0.00%> (ø)
ballista/rust/executor/src/flight_service.rs	0.00% <0.00%> (ø)
ballista/rust/executor/src/main.rs	0.00% <0.00%> (ø)

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