Fixed Authenticator SASL auxiliary memory access.

Review: https://reviews.apache.org/r/27741
Added slave metrics for total and used resources.

Review: https://reviews.apache.org/r/27759
Fixed invalid characters in external-containerizer.md

Review: https://reviews.apache.org/r/27724
Added check for apr and svn headers besides libraries

Review: https://reviews.apache.org/r/27701
Added check for apr and svn headers besides libraries in 3rdparty

Review: https://reviews.apache.org/r/27704
Always recover 'completed' in RunState.

Review: https://reviews.apache.org/r/27738
Add to 0.21.0 CHANGELOG.
Fixes YAML header in Mesos containerizer doc.
Fixed scheduler driver to not acknowledge status update when stop() is called.

Review: https://reviews.apache.org/r/27896
Added missing ZooKeeper patch file to the Makefile.
Update CHANGELOG for 0.21.0-rc3
Modified libprocess configure.ac to set $with_svn to `brew --prefix subversion` for OS X.

Review: https://reviews.apache.org/r/28722
Modified mesos configure.ac to set $with_svn to `brew --prefix subversion` for OS X.

Review: https://reviews.apache.org/r/28721
Do not return error if removing bind mount fails.

Review: https://reviews.apache.org/r/28339
Made libprocess generate valid HTTP.

While working on a pure Scala driver for Mesos, I discovered that
libprocess does not generate valid HTTP. It uses chunked encoding
combined with HTTP 1.0, which has only been added in HTTP
1.1. Additionally it stores the PID in the User-Agent field with an
incompatibe format ('@' and ':' are not allowed). This patch sets the
HTTP version to 1.1, adds an empty Host header (Host is mandatory in
1.1) and adds the Libprocess-From header. The User-Agent header is
left untouched for backwards compatibility reasons. Even strict HTTP
parser shouldn't error, but just ignore everything between the invalid
character and the CRLF.

Review: https://reviews.apache.org/r/27865
Allowed turning off network socket statistics collection.

Review: https://reviews.apache.org/r/28295
Destroy Docker containers with stop instead of kill.

Ensure docker calls stop if a docker_stop_timeout is provided.
Added the flag docker_stop_timeout that defaults to 0, if the
timeout is 0 the a docker kill will be run, otherwise a docker stop
with the timeout in seconds is used.

Review: https://reviews.apache.org/r/26736
Update CHANGELOG for 0.21.1
Update configure.ac for 0.21.1
Updated docs/configuration.md.

- Copied the output of --help messages from configure, master, and
  slave.
- The option tables are now in html format.
- Removed the explanation of the default configure flags.

Review: https://reviews.apache.org/r/27556