Convert thrifty-runtime to kotlin, require Java 8

[benjamin-bader]

Closes #387
Closes #388

[benjamin-bader]

Breaking change: callbacks for oneway service methods now take Unit, not Void.

[codecov-commenter]

Codecov Report

Merging #391 into master will decrease coverage by 1.52%.
The diff coverage is 71.02%.

@@             Coverage Diff              @@
##             master     #391      +/-   ##
============================================
- Coverage     62.85%   61.33%   -1.53%     
+ Complexity     1116     1039      -77     
============================================
  Files            83       84       +1     
  Lines          7000     6711     -289     
  Branches       1004     1011       +7     
============================================
- Hits           4400     4116     -284     
+ Misses         2258     2251       -7     
- Partials        342      344       +2     

Impacted Files	Coverage Δ	Complexity Δ
...kotlin/com/microsoft.thrifty.gen/ServiceBuilder.kt	0.45% <0.00%> (ø)	1.00 <0.00> (ø)
.../com/microsoft/thrifty/kgen/KotlinCodeGenerator.kt	81.28% <0.00%> (ø)	135.00 <0.00> (ø)
...ime/src/main/kotlin/com/microsoft/thrifty/TType.kt	0.00% <0.00%> (ø)	0.00 <0.00> (?)
...tlin/com/microsoft/thrifty/service/TMessageType.kt	0.00% <0.00%> (ø)	0.00 <0.00> (?)
...kotlin/com/microsoft/thrifty/service/MethodCall.kt	33.33% <33.33%> (ø)	2.00 <2.00> (?)
...kotlin/com/microsoft/thrifty/service/ClientBase.kt	49.05% <49.05%> (ø)	7.00 <7.00> (?)
.../com/microsoft/thrifty/protocol/MessageMetadata.kt	50.00% <50.00%> (ø)	1.00 <1.00> (?)
...com/microsoft/thrifty/transport/BufferTransport.kt	50.00% <50.00%> (ø)	3.00 <3.00> (?)
...n/com/microsoft/thrifty/service/AsyncClientBase.kt	55.55% <55.55%> (ø)	7.00 <7.00> (?)
...m/microsoft/thrifty/protocol/SimpleJsonProtocol.kt	65.38% <65.38%> (ø)	25.00 <25.00> (?)
... and 35 more

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[benjamin-bader]

More breaking changes:

• Protocol is now an interface
• Transport is now an interface