use new buffers by default

[Mythra]

Description:

switch Envoy::Buffer to Brian Pane's implementation (aka "new buffers") by default.

Risk Level: Medium

• There's been decent testing on this, but it's still a pretty massive change.

Testing:

Ran through the following test plan locally:

1. Build envoy on this PR.
2. Spin up a server with no CLI Args: `./bazel-bin/source/exe/envoy-static -c /tmp/config.yaml` validate that: `[2019-08-09 10:11:52.900][6365][info][main] [source/server/server.cc:269] buffer implementation: new` gets logged
3. Ensure new buffers can be turned off by running: `./bazel-bin/source/exe/envoy-static --use-libevent-buffers 1 -c /tmp/config.yaml` validate that: `[2019-08-09 10:12:14.247][6431][info][main] [source/server/server.cc:269] buffer implementation: old (libevent)` got logged

There's also be some testers in actual environments see #7314 , and Envoy Meeting Minutes.

Docs Changes: None
Release Notes: Added a line about how it's the default, including how to turn it off.

fixes #7314

[Mythra]

@lizan ,

ince @jmarantz is OOO any chance you could review this as a senior maintainer? I'd like to get it out before the security releases tomorrow as well so I can pick these onto our local branch along with the security fixes, it has been approved as of the latest envoy meeting.

[lizan]

@securityinsanity sure, the code looks good to me, however I'm not merging this until the security release, to keep the upgrade process smooth.

[Mythra]

👍 I agree there, our pipeline just checks for approved PRs before picking.

Thanks!

[Mythra]

I believe we should be good to merge this now, since 1.11.1 is fully out?

[mattklein123]

🤞 please merge master.

/wait

[Mythra]

Can do here in a bit.

[Mythra]

Master has been merged, successfully 👍 .

[mattklein123]

Thanks!