[BEAM-115,BEAM-1348] Unify Runner API and Fn API coders

[kennknowles]

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Just shares Coder. Its a hack - it is really crufty to have side-by-side int and string registries. But coders need to be by-reference. Options, off the top of my head:

1. Make everything keyed on int64 (distasteful for debugging, but not horrid).
2. Make everything keyed on string.
3. Make Fn API and harness really embrace separate registries by type.
4. Write 3 lines of conversion code, since sharing this one trivial proto has only unclear benefit, while avoiding a hard dependency is almost always a win.

R: @dhalperi

[asfbot]

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Build result: FAILURE

[...truncated 1.05 MB...] at hudson.remoting.UserRequest.perform(UserRequest.java:153) at hudson.remoting.UserRequest.perform(UserRequest.java:50) at hudson.remoting.Request$2.run(Request.java:336) at hudson.remoting.InterceptingExecutorService$1.call(InterceptingExecutorService.java:68) at java.util.concurrent.FutureTask.run(FutureTask.java:266) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617) at java.lang.Thread.run(Thread.java:745)Caused by: org.apache.maven.plugin.MojoExecutionException: Dependency problems found at org.apache.maven.plugins.dependency.analyze.AbstractAnalyzeMojo.execute(AbstractAnalyzeMojo.java:261) at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo(DefaultBuildPluginManager.java:134) at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:208) ... 31 more2017-02-22T03:55:24.192 [ERROR] 2017-02-22T03:55:24.192 [ERROR] Re-run Maven using the -X switch to enable full debug logging.2017-02-22T03:55:24.192 [ERROR] 2017-02-22T03:55:24.192 [ERROR] For more information about the errors and possible solutions, please read the following articles:2017-02-22T03:55:24.192 [ERROR] [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/MojoExecutionException2017-02-22T03:55:24.193 [ERROR] 2017-02-22T03:55:24.193 [ERROR] After correcting the problems, you can resume the build with the command2017-02-22T03:55:24.193 [ERROR] mvn -rf :beam-sdks-java-harnesschannel stoppedSetting status of 3d4aed8 to FAILURE with url https://builds.apache.org/job/beam_PreCommit_Java_MavenInstall/7675/ and message: 'Build finished. 'Using context: Jenkins: Maven clean install
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[coveralls]

Coverage decreased (-0.01%) to 69.287% when pulling 6c5ec75 on kennknowles:fn-api-coders-only into 0d639e6 on apache:master.

[asfbot]

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--none--

[kennknowles]

I'm pretty unhappy with the cruftiness of this, so now that I've proposed a few unification possibilities, I'm going to rest easy assuming there won't be any or much sharing. I won't yet collapse and simplify GraphNode since it isn't blocking.