Fixed slave to properly do memory isolation when executor resources
are not specified.

From: Vinod Kone <vinodkone@gmail.com>
Review: https://reviews.apache.org/r/17295
Refined http get/post interfaces to use Option.

From: Jie Yu <yujie.jay@gmail.com>
Review: https://reviews.apache.org/r/17076
Updated http tests in libprocess for the new get/post interfaces.

From: Jie Yu <yujie.jay@gmail.com>
Review: https://reviews.apache.org/r/17077
Refactored implicit conversions of Some, None, and Error.

Review: https://reviews.apache.org/r/17089
Added a conversion constructor to Future.

Review: https://reviews.apache.org/r/17090
Added overload of numify.

Review: https://reviews.apache.org/r/17091
Added some missing includes.

Review: https://reviews.apache.org/r/17092
Bug in http::request missing os::close.

Review: https://reviews.apache.org/r/17093
Made proposal number optional in log::catchup.

From: Jie Yu <yujie.jay@gmail.com>
Review: https://reviews.apache.org/r/17339
Allowed log::catchup to retry to tolerate network blips.

In case of a network blip (zookeeper network), log::catchup could get
stuck even if all replicas reemerge later. This is not ideal for some
users such as log::recover.

So I introduced an optional timeout for log::catchup so that the
catchup operation on each position will be retried if timeout happens.

From: Jie Yu <yujie.jay@gmail.com>
Review: https://reviews.apache.org/r/17286
Added Result constructor which takes Option.
Added launchTasks for list of offers

Running tasks on more than one offer belonging to a single slave can
be useful in situations with multiple out-standing offers.

This patch extends the usual launchTasks() to accept a vector of
OfferIDs. The previous launchTasks (accepting a single OfferID) has
been kept for backward compatibility, but this now calls the new
launchTasks() with a one-element list. This also applied for the JNI
and python interfaces, which accepts both formats as well.

Offers are verified to belong to the same slave and framework,
before resources are merged and used.

Review: https://reviews.apache.org/r/14669
Changed catchup.cpp to make it compilable with clang and C++11.

From: Jie Yu <yujie.jay@gmail.com>
Review: https://reviews.apache.org/r/17435
Merge branch 'master' into feature/Debian
[Deb] --oknodo prevent upgrade from failing
[deb] bump 0.17.0-4