Make Repeater and Streamloader more consistent with each other

[dilyand]

These two applications do a lot of things that are the same, eg reading from Pub/Sub and writing to BigQuery. However, the implementations are very different.

Just one example: #184 (comment)

[istreeter]

Issue #217 is also concerned with the repeater and streamloader. But I deliberately made it a new issue because I think it's important enough. This issue is more about implementation details, whereas #217 is virtually a bug.