pubsub: set keepalive
pubsub: set keepalive

Set keepalive so that idle stream does not make us hang
indefinitely.

Fixes #740.

Change-Id: I925443475a98fe860e891e7117a2e229819866df
Reviewed-on: https://code-review.googlesource.com/17070
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go