bigtable: Use connection pool by default for data client.
bigtable: Use connection pool by default for data client.

This makes it less likely that users will be throttled with low qps,
and more likely that a retry will go to a different connection which helps if
a particular connection is having issues. Pool size can still be overridden.

Change-Id: I975e6bfcaedfccf5f9e3b72dd9d3d36f05be963d
Reviewed-on: https://code-review.googlesource.com/8930
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,michael-grosshans/google-cloud-go,michael-grosshans/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go