uhd: Replace Boost mutexes and locks with standard options

This is a very mechanical task that could almost have been done with
sed. Boost versions of mutexes and locks were removed, and replaced with
std:: versions. The replacement tables are as follows:

== Mutexes ==
- boost::mutex -> std::mutex
- boost::recursive_mutex -> std::recursive_mutex

Mutexes behave identically between Boost and std:: and have the same
API.

== Locks ==
C++11 has only two types of lock that we use/need in UHD:
- std::lock_guard: Identical to boost::lock_guard
- std::unique_lock: Identical to boost::unique_lock

Boost also has boost::mutex::scoped_lock, which is a typedef for
boost::unique_lock<>. However, we often have used scoped_lock where we
meant to use lock_guard<>. The name is a bit misleading, "scoped lock"
sounding a bit like an RAII mechanism. Therefore, some previous
boost::mutex::scoped_lock are now std::lock_guard<>.

std::unique_lock is required when doing more than RAII locking (i.e.,
unlocking, relocking, usage with condition variables, etc.).

== Condition Variables ==
Condition variables were out of the scope of this lock/mutex change, but
in UHD, we inconsistently use boost::condition vs.
boost::condition_variable. The former is a templated version of the
latter, and thus works fine with std::mutex'es. Therefore, some
boost::condition_variable where changed to boost::condition.

All locks and mutexes use `#include <mutex>`. The corresponding Boost
includes were removed. In some cases, this exposed issues with implicit
Boost includes elsewhere. The missing explicit includes were added.

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