Switched path::join() to be variadic

Review: https://reviews.apache.org/r/25818
MESOS-1857 Fixed path::join() on older libstdc++ which lack back().

Review: https://reviews.apache.org/r/26256
Added webui_url to FrameworkInfo and web UI.

Allows frameworks to register a URL to its web UI.

Review: https://reviews.apache.org/r/26125
Replace some asserts with CHECKs in dispatch.

Review:
Added TCP RTT statistics for port mapping network isolator.

Review: https://reviews.apache.org/r/26090
Converted diagnosis socket Info to a struct.

Review: https://reviews.apache.org/r/26290
Moved Mesos version from mesos.hpp to version.hpp.

This allows the Module subsystem to simply include <mesos/version.hpp>
instead of inheriting the whole protobuf header chain.

Review: https://reviews.apache.org/r/26303
Install Python libraries during make install.

Because eggs are impossible to uninstall, pip + wheel has been added
as a packaging format. The wheels are generated in addition to eggs
and then subsequently installed/uninstalled.

Note that the building of eggs has not been removed. This is because
wheels cannot be run in place like eggs are via. modification of
PYTHONPATH. The eggs are needed for all the test scripts to run
correctly without actually installing anything locally.

Review: https://reviews.apache.org/r/24264
Split messages_launch_tasks into messages_launch_tasks and messages_decline_offers.

Review: https://reviews.apache.org/r/26329
Fixed ~Authenticator() to discard the promise.

Review: https://reviews.apache.org/r/26389
Fixed a resource accounting performance regression in the Master.

Review: https://reviews.apache.org/r/26392
Short term fix for MESOS-1866.

Review: https://reviews.apache.org/r/26400
Add --with-curl to libprocess since <stout/net.hpp> needs it.

libprocess uses `<stout/net.hpp>` which includes `<curl/curl.h>`. On a machine without system-level libcurl installed, we should be able to build `mesos` with the `--with-curl=<path-to-libcurl>` flag. The `-I<path-to-libcurl>` gets added to the `CPPFLAGS` for `mesos` but it doesn't added for libprocess. This patch makes libprocess actually use the `--with-curl` flag.

Review: https://reviews.apache.org/r/26352
Changed checkpoint logging from LOG(INFO) to VLOG(1).

Review: https://reviews.apache.org/r/26242
Fixed a log line in the master.

Review: https://reviews.apache.org/r/26198
Eliminated redundant resource accounting in the master.

Review: https://reviews.apache.org/r/26199
Added validation for missing ExecutorInfo::framework_id.

Review: https://reviews.apache.org/r/26200
Properly deprecated ReregisterSlaveMessage::slave_id.

Review: https://reviews.apache.org/r/26201
Introduced a version during slave (re-)registration.

Review: https://reviews.apache.org/r/26202
Removed redundant logging in the Master.

Review: https://reviews.apache.org/r/26203