WIP: make ClientContext container-only, never initializing

[pongad]

WIP pending more tests,
but should be good enough for review.
All unit tests pass against both old and regenerated GCJ
but we should integ to make sure.

Adding Neo since a related change broke his code earlier.

[codecov-io]

Codecov Report

Merging #318 into master will increase coverage by 0.3%.
The diff coverage is 70%.

@@             Coverage Diff             @@
##             master     #318     +/-   ##
===========================================
+ Coverage     69.81%   70.11%   +0.3%     
- Complexity      355      358      +3     
===========================================
  Files            73       73             
  Lines          1925     1921      -4     
  Branches        157      158      +1     
===========================================
+ Hits           1344     1347      +3     
+ Misses          513      506      -7     
  Partials         68       68

Impacted Files	Coverage Δ	Complexity Δ
...va/com/google/api/gax/grpc/ChannelAndExecutor.java	75% <ø> (+75%)	3 <0> (+3)	⬆️
.../google/api/gax/core/FixedCredentialsProvider.java	0% <ø> (ø)	0 <0> (ø)	⬇️
...gle/api/gax/grpc/InstantiatingChannelProvider.java	47% <0%> (-0.96%)	9 <0> (ø)
...in/java/com/google/api/gax/grpc/ClientContext.java	100% <100%> (+14.28%)	2 <1> (-1)	⬇️
...n/java/com/google/api/gax/grpc/ClientSettings.java	48.97% <33.33%> (+2.04%)	5 <0> (+1)	⬆️
.../java/com/google/longrunning/OperationsClient.java	93.54% <90%> (-1.91%)	17 <0> (ø)

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[garrettjonesgoogle]

LGTM

[pongad]

I tested this against both current GCJ and the one generated by googleapis/gapic-generator#1319. Each test includes mvn test on all projects and mvn verify on pubsub.

I'll merge this, but hold off on toolkit until a gax version bump.