…tion_variable (fix ros#1343)

ros#1014 and ros#1250 introduced a backported
version of boost::condition_variable, where support for steady (monotonic) clocks has been added in version 1.61.
But the namespace of the backported version was not changed and the symbol names might clash with the original
version.

Because the underlying clock used for the condition_variable is set in the constructor and must be
consistent with the the expectations within member variables. The compiler might choose to inline one or the
other or both, and is more likely to do so for optimized Release builds. But if it does not, the symbol ends
up in the symbol table of roscpp and depending on which other libraries will be linked into the process it
is unpredictable which of the two versions will be actually called at the end. In case the constructor defined
in `/usr/include/boost/thread/pthread/condition_variable.hpp` was called and did not configure the internal
pthread condition variable for monotonic clock, each call to the backported do_wait_until() method with a
monotonic timestamp will return immediately and hence causes `CallbackQueue::callOne(timeout)` or
`CallbackQueue::callAvailable(timeout)` to return immediately.

This patch changes the namespace of the backported condition_variable implementation to boost_161. This
removes the ambiguity with the original definition if both are used in the same process.

…ed timed_wait()

This fixes ROS timers in combination with 2c18b9f. The timer
callbacks were not called because the TimerManager's thread function blocked indefinitely on
boost::condition_variable::timed_wait().

Relative timed_wait() uses the system clock (boost::get_system_time()) unconditionally to
calculate the absolute timestamp for do_wait_until(). If the condition variable has been
initialized with BOOST_THREAD_HAS_CONDATTR_SET_CLOCK_MONOTONIC, it compares this timestamp
with the monotonic clock and therefore blocks.

This issue has been reported in https://svn.boost.org/trac10/ticket/12728 and will not be
fixed. The timed_wait interface is apparently deprecated.

…eadFunc() in steady_timer.cpp

The updated generic definition in timer_manager.h should do the same with a minor update.
In all cases we can call boost::condition_variable::wait_until() with an absolute time_point of the respective clock.
The conversion from system_clock to steady_clock for Time and WallTime is done internally in
boost::condition_variable::wait_until(lock_type& lock, const chrono::time_point<Clock, Duration>& t).

find_package(Boost) has to come before checking the Boost version.
Otherwise BOOST_THREAD_HAS_CONDATTR_SET_CLOCK_MONOTONIC was not defined which
triggered the assertion in timer_manager.h:240.

Since Boost 1.67 BOOST_THREAD_HAS_CONDATTR_SET_CLOCK_MONOTONIC became the default
if the platform supports it and the macro is not defined anymore. Instead, check
for BOOST_THREAD_INTERNAL_CLOCK_IS_MONO.

…ITION_VARIABLE_HEADER macros by a typedef in internal_condition_variable.h

…61 in namespace boost_161

…nimize the diff to melodic-devel

Addresses ros#1878 (review).

This reverts commit 2a67cf6.