BigQuery Job ID Generation

[jdpedrie]

This change introduces client-generated Job IDs. It also adds support for user-specified job prefixes.

The goal of this feature is to make inserting jobs idempotent. I am concerned that I may not have quite captured that in this pull request. Job IDs are generated when BigQueryClient::runQueryAsJob() or the various Job-creating methods on Table are called. This means that multiple calls to those methods will result in different unique Job IDs, even if the configuration is identical.Those IDs are accessible after the call is executed, by invoking Job::id(). Currently there is not a means for a user to manually specify a Job ID (only a prefix); this means that one cannot retry with an existing Job ID.

If this is an issue, it could be mitigated without major changes to the client by adding an optional $jobId argument to the various job-creating methods' $options, along with documentation advising against using it except in well-defined circumstances.

[jdpedrie]

phpunit is getting hung up on a spanner unit test:

Starting test 'Google\Cloud\Tests\Unit\Spanner\Connection\GrpcTest::testCallBasicMethods with data set #0 ('listInstanceConfigs', array('projects/my-project'), array('projects/my-project', array(array(array('projects/my-project')))))'.

I'm investigating now!

[jdpedrie]

test issue temporarily resolved by #624.

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