Added option to force pull docker image.

Review: https://reviews.apache.org/r/28190
Fixed a few grammar mistakes in mesos.proto.
Silenced symbolic link to pre-commit in bootstrap.

Fixes MESOS-1960.

Review: https://reviews.apache.org/r/28303
Added validation for Resource objects with DiskInfo.

Review: https://reviews.apache.org/r/28258
Allowed C++ Resources to handle DiskInfo.

Review: https://reviews.apache.org/r/28264
Fixed a bug due to missing canonicalize.

Review: https://reviews.apache.org/r/28304
Added missed changes to DiskInfo patches..
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
Fix ProcTest.MultipleThreads hanging.

Replace 'sleep' with 'pthread_testcancel'.

Review: https://reviews.apache.org/r/28137
Fix a race between SocketManager link() and send().

Review: https://reviews.apache.org/r/27967
Factor out Future callback invocation.

Factor out callback invocation in Future to make the logic easier to
read. It also de-duplicates some code.

Review: https://reviews.apache.org/r/28195
Replace std::queue with std::vector in Libprocess Future.

Use std::vector instead of std::queue to reduce dynamic allocations
and improve performance.

Review: https://reviews.apache.org/r/28196
Do not return error if removing bind mount fails.

Review: https://reviews.apache.org/r/28339
Set memory::limit_in_bytes first, even with limitSwap enabled.

Review: https://reviews.apache.org/r/28305
Changed RunTaskWithCommandInfoWithUser to not use shell.

The 'nobody' user is restricted on some systems from shell access and
broke the task.user tests (which have been disabled since then).  With
the command.shell field, the test can run the command directly.

Review: https://reviews.apache.org/r/26894
Allow mesos containerizer to destroy correctly at each state.

Review: https://reviews.apache.org/r/28141
Fix style issue for mesos containerizer commit.
Allowed slave to checkpoint resources.

Review: https://reviews.apache.org/r/26990
Fixed a compilation error on gcc-4.4.

Review: https://reviews.apache.org/r/28357
Fix deadlock between SocketManager::send() and internal::send().

This was a deadlock between the 'this' and 'processes' synchronized
blocks that occured because the .then() on socket futures can be
invoked immediately with the new implementation of io::poll().

Review: https://reviews.apache.org/r/28367