requests.post() creates a new HTTP connection for each call.
It makes presubmit checks slower and timed out.
This CL updates rdb_wrapper to use requests.Session() to use
a connection pool for Sink requests.

Change-Id: Id02ecce85898ae15993d53df17473bb5dfb7e89f
Bug: 1145762
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2532895
Commit-Queue: Scott Lee <ddoman@chromium.org>
Reviewed-by: Chan Li <chanli@chromium.org>
Reviewed-by: Erik Staab <estaab@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>