Move Clock out of ServiceOptions, use it in RetryHelper

[mziccard]

This fixes #950. This also moves Clock out of ServiceOptions as it makes more sense since it's shared with RetryHelper.

[coveralls]

Coverage increased (+0.02%) to 85.757% when pulling 329ab44 on mziccard:retry-helper-clock into 0a6c6f7 on GoogleCloudPlatform:master.

[aozarov]

Looks great. Next we need to use that Clock in all our RetryHelper usages (and make runWithRetries with Clock public).

[mziccard]

Before merging this we should make:

runWithRetries(Callable<V> callable, RetryParams params, ExceptionHandler exceptionHandler, Clock clock)

public and use it across all services passing the clock contained in ServiceOptions.

[coveralls]

Coverage increased (+0.007%) to 85.982% when pulling 497ccbd on mziccard:retry-helper-clock into d5cf726 on GoogleCloudPlatform:master.

[mziccard]

Rebased to get compute's module. Added 497ccbd to pass options.clock() to retryHelper.runWithRetries everywhere we use it.