Fixed control flow compiler warning in encoder.hpp.

Fixes warning of "control may reach end of non-void function" in
encoder.hpp that occurs when compiling using clang.

From: TILL TOENSHOFF <toenshoff@me.com>
Review: https://reviews.apache.org/r/16897
Fixed omitted includes.

The includes of the respective C++11 standard headers were
missing. This issue becomes visible only when including one of those
headers before including a standard C++ header. As the namespace std::
simply does not exist without including a standard header, these two
headers produced errors on clang.

From: TILL TOENSHOFF <toenshoff@me.com>
Review: https://reviews.apache.org/r/16912
Added filtering to all tables.

From: Thomas Rampelberg <thomas@saunter.org>
Review: https://reviews.apache.org/r/16851
When the detectors fail, subsequent detect() calls now get failed
Futures directly.

From: Jiang Yan Xu <yan@jxu.me>
Review: https://reviews.apache.org/r/16586
Made ZooKeeperMasterContenderProcess::master an option to codify the
fact that it can be unset after construction.

From: Jiang Yan Xu <yan@jxu.me>
Review: https://reviews.apache.org/r/16588
ZooKeeperMasterContenderProcess::contend now checks if the future for
the previous contend() is still pending and if so it directly returns
that future without recontending.

From: Jiang Yan Xu <yan@jxu.me>
Review: https://reviews.apache.org/r/16589
LeaderContender::withdraw() now always returns false if there is no
membership to withdraw.

This includes when:
- withdraw() is called before contend()
- withdraw() is called after contend() but before the contender learns that it fails to obtain the candidacy
- withdraw() is called after the contender has learned that it had failed to obtain the candidacy

From: Jiang Yan Xu <yan@jxu.me>
Review: https://reviews.apache.org/r/16590
GroupProcess::abort() now also sets all owned memberships as cancelled.

From: Jiang Yan Xu <yan@jxu.me>
Review: https://reviews.apache.org/r/16591
Added an option to StartMaster() wait for leader election result before
returning.

From: Jiang Yan Xu <yan@jxu.me>
Review: https://reviews.apache.org/r/16742