LTS backport: normalize httplib2.Http initialization and usage

Ideally solve both issues below in one PR, but issue 1 has priority as it can halt a pipeline.

Issue 1:
Datastore client (and other httplib2-based clients for GCS, Dataflow, BigQuery, etc.) doesn't set a socket timeout.
This can cause _flush_batch() in datastoreio.py to block forever waiting for a response.

This issue is very similar to https://issues.apache.org/jira/browse/BEAM-5915 and the solution should be similar.

Issue 2:
Standardize use of proxy environment settings, as in gcsio:
https://github.com/apache/beam/blob/8d3389df78aa2e0a0de06b7c5743ca3530dec4ac/sdks/python/apache_beam/io/gcp/gcsio.py#L136

Issue for proxy settings: https://issues.apache.org/jira/browse/BEAM-3184