Splitting GAX into gax + gax-grpc

[garrettjonesgoogle]

Tested:

• build/test/install
• license validation
• google-java-format: there is a problem with this plugin, so I filed an issue to address it later (Fix running of google-java-format in individual modules #287); basically, gax-grpc won't be covered by the plugin for now.
• updateDocsWithCurrentVersion (using distinct gax/gax-grpc versions)
• uploadArchives (using distinct gax/gax-grpc versions) (checked in Sonatype)

Note: The version has to be bumped with this commit, otherwise the README will point to a non-existent package.

Still to do

• make an update in google-cloud-java for these changes

[codecov-io]

Codecov Report

Merging #288 into master will decrease coverage by 2.89%.
The diff coverage is 50%.

@@ Coverage Diff @@## master #288 +/- ##
===========================================
- Coverage 73.78% 70.88% -2.9% + Complexity 361 341 -20 
===========================================
Files 66 68 +2 Lines 1812 1817 +5 Branches 148 148 ===========================================
- Hits 1337 1288 -49 - Misses 405 464 +59 + Partials 70 65 -5

Impacted Files	Coverage Δ	Complexity Δ
.../java/com/google/api/gax/grpc/OperationFuture.java	80% <ø> (ø)	16 <0> (?)
...le/api/gax/retrying/ScheduledRetryingExecutor.java	100% <ø> (ø)	3 <0> (?)
...va/com/google/api/gax/grpc/ChannelAndExecutor.java	75% <ø> (ø)	3 <0> (?)
.../java/com/google/api/gax/grpc/ProviderManager.java	0% <ø> (ø)	0 <0> (?)
...om/google/api/gax/grpc/UnaryCallSettingsTyped.java	88.88% <ø> (ø)	3 <0> (?)
...le/api/gax/grpc/InstantiatingExecutorProvider.java	87.5% <ø> (ø)	4 <0> (?)
...ava/com/google/api/gax/grpc/HeaderInterceptor.java	100% <ø> (ø)	3 <0> (?)
...gle/api/gax/grpc/testing/FakeMethodDescriptor.java	50% <ø> (ø)	2 <0> (?)
...java/com/google/api/gax/grpc/BatchingCallable.java	88.88% <ø> (ø)	3 <0> (?)
...ogle/api/gax/grpc/AbstractFixedSizeCollection.java	75% <ø> (ø)	10 <0> (?)
... and 71 more

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[garrettjonesgoogle]

@michaelbausor I'm putting this out for review now even though the google-cloud-java PR isn't ready, so that the completion of the full review can be earlier.

[michaelbausor]

Initial review looks good to me

[michaelbausor]

LGTM