Updates #PoweredByMesos list to include DueDil.
Revised the handling of truncation in leveldb to address the issue exposed by log catch-up.
Added log storage test for testing truncation.
Updated the CHANGELOG for 0.18.0-rc2.
Fixed comparator in tasks endpoint.

Related to MESOS-979, the tasks.json endpoint would sporadically cause the
master process to SEGFAULT. The code failed during the subsequent sort
(after gathering running and completed tasks) deferencing garbage pointer
values. The issue turns out to be an issue with std::sort and strict weak-
ordering. Descending order (which was default) was implemented as !ascending()
which is not a strict greather-than equal. This patch expands the descending
implementation and reenables endpoint.

Review: https://reviews.apache.org/r/18138
Change stout JSON::Value default to be JSON::Null.

The first type specified for the variant which is JSON::Value is the
default. Currently that is JSON::String which in turn defaults to
empty string. JSON::Null would be a more correct default value.

This change moves JSON::Null to the front of the declaration so that
it becomes the default and adds a unit test.

Review: https://reviews.apache.org/r/17568
Move process::http namespace into http.cpp.

This moves the code declared in the process::http namespace to be in
it's own file (http.cpp). It already had it's own header.

Bug: https://issues.apache.org/jira/browse/MESOS-936

Review: https://reviews.apache.org/r/17341