Sort out BundlingSettings

[pongad]

@garrettjonesgoogle @davidtorres

From our meeting last Thursday, I think the decision was

• These fields should be set and non-zero
  • elementCountThreshold
  • requestByteThreshold
  • delayThreshold
• Delete the rest

I wanted to make sure I get it right to avoid code churn since this is tightly coupled with work in google-cloud-java.

[davidtorres]

SGTM

[garrettjonesgoogle]

One more item needs to be supported: blocking call count threshold. With that added, LGTM.