ARROW-4625: [Flight][Java] Add method to await Flight server termination in Java

[ghost]

Apologies for all the tiny PRs...trying to clean up some small API things we've run into.

[codecov-io]

Codecov Report

Merging #4110 into master will increase coverage by 0.81%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4110      +/-   ##
==========================================
+ Coverage   87.82%   88.63%   +0.81%     
==========================================
  Files         739      604     -135     
  Lines       90965    81304    -9661     
  Branches     1252        0    -1252     
==========================================
- Hits        79886    72065    -7821     
+ Misses      10958     9239    -1719     
+ Partials      121        0     -121

Impacted Files	Coverage Δ
go/arrow/math/uint64_amd64.go
go/arrow/memory/memory_avx2_amd64.go
js/src/enum.ts
go/arrow/array/builder.go
js/src/Arrow.node.ts
js/src/schema.ts
go/arrow/type_traits_boolean.go
js/src/ipc/node/writer.ts
js/src/visitor/vectorloader.ts
js/src/visitor.ts
... and 128 more

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[wesm]

+1