[BEAM-167] Fix custom source gzip input to read concatenated gzip files

This applies patch from kirpichov@google.com from https://gist.github.com/jkff/d8d984a33a41ec607328cee8e418c174

Merge pull request GoogleCloudPlatform#146 from kennknowles/backport-…

…pr-38

Backport apache/beam#38

Fixes a bug in custom unbounded readers

Custom unbounded readers are read in bundles of at most
10k elements or 10 seconds. A recent change accidentally removed
the 10k element limit. This change reintroduces it and
adds a test.

The previous test also was passing vacuously because
the iteration limit was incorrect (it would always
have only one iteration).
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=112723469

Occasionally compact combined state on write.

This is needed to avoid arbitrarily large blowup
for many-element windows.  Now, with a fixed
probability (currently 0.2%) when writing the state,
instead of doing a blind write we read in all
accumulated values, combine, and replace everything
with the final combined value.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=109239045

Version management

Declare version 1.2.0. Prepare for version 1.2.1.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=104700491

Handle exceptions from within the SDK while processing work

The DataflowWorkerHarness when processing work wasn't handling
general exceptions from being thrown causing the processing
threads to all die leaving a zombie worker.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=104664789

Updates dependency google-api-services-datastore-protobuf.

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Updating dependency google-api-services-datastore-protobuf from v1beta2-rev1-2.1.2 to v1beta2-rev1-3.0.2.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=103114238

Dataflow GA: update GitHub's README.md to remove Beta language

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Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=100331508

Move GroupByKey validation into validate method

----Release Notes----
Re-enabled verification of GroupByKey usage. Specififically, the key
must have a deterministic coder and using GroupByKey with an Unbounded
PCollection requires windowing or triggers.

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Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=99194010

Version 0.4.20150710