Concurrent in-flight requests to Stackdriver Logging

[fluffle]

Thanks to some great work by @jba we can now have multiple in-flight requests to Stackdriver. This should make Nozzle throughput much less susceptible to latency-induced drops.

This PR pulls in the vendor changes and adds a new config option so that the concurrency limit is tuneable (default: 16).

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This change is 

[knyar]

Looks reasonable, but let's wait for @johnsonj to lgtm.

[knyar]

Reviewed 11 of 11 files at r1.
Review status: all files reviewed at latest revision, 1 unresolved discussion.

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jobs/stackdriver-nozzle/spec, line 56 at r1 (raw file):

    default: 30

  nozzle.logging_requests_in_flight

Colon missing here.

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Comments from Reviewable

[johnsonj]

Reviewed 11 of 11 files at r1.
Review status: all files reviewed at latest revision, 1 unresolved discussion.

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Comments from Reviewable

[johnsonj]

pending Anton's comment

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Review status: all files reviewed at latest revision, 1 unresolved discussion.

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Comments from Reviewable

[fluffle]

This "stale LGTM" feature really breaks the "LGTM once you do X, Y, Z" work pattern that is basically necessary for reasonable cross-continent development velocity. I wonder if there's any way of telling Reviewable to STFU once an LGTM is given.

[fluffle]

Review status: 10 of 11 files reviewed at latest revision, 1 unresolved discussion.

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jobs/stackdriver-nozzle/spec, line 56 at r1 (raw file):

Previously, knyar (Anton Tolchanov) wrote…

Colon missing here.

Done.

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Comments from Reviewable

[knyar]

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Reviewed 1 of 1 files at r2.
Review status: all files reviewed at latest revision, 1 unresolved discussion.

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Comments from Reviewable