Import cpp-btree library into 3rdparty section.

Repo: https://github.com/JGRennison/cpp-btree
Commmit: 6cabdb40fcbb7e12e6d499f92b898f6ec80ae0f1

Remove test program, build scripts, etc.

src/3rdparty/cpp-btree/README

@@ -0,0 +1,45 @@
+This library is a C++ template library and, as such, there is no
+library to build and install.  Copy the .h files and use them!
+
+See http://code.google.com/p/cpp-btree/wiki/UsageInstructions for
+details.
+
+----
+
+To build and run the provided tests, however, you will need to install
+CMake, the Google C++ Test framework, and the Google flags package.
+
+Download and install CMake from http://www.cmake.org
+
+Download and build the GoogleTest framework from
+http://code.google.com/p/googletest
+
+Download and install gflags from https://code.google.com/p/gflags
+
+Set GTEST_ROOT to the directory where GTEST was built.
+Set GFLAGS_ROOT to the directory prefix where GFLAGS is installed.
+
+export GTEST_ROOT=/path/for/gtest-x.y
+export GFLAGS_ROOT=/opt
+
+cmake . -Dbuild_tests=ON
+
+For example, to build on a Unix system with the clang++ compiler,
+
+export GTEST_ROOT=$(HOME)/src/googletest
+export GFLAGS_ROOT=/opt
+cmake . -G "Unix Makefiles" -Dbuild_tests=ON -DCMAKE_CXX_COMPILER=clang++
+
+----
+
+Note that this is a modified version of the original at http://code.google.com/p/cpp-btree
+
+Changes include:
+- Changing size_type from ssize_t to size_t
+- Adding cbegin, cend, crbegin, crend.
+- Adding key_comp
+- Adding move constructors/assignment.
+- No longer #define NDEBUG if unset, this can clash with other headers, test for BTREE_DEBUG being defined instead
+
+CMakeLists-pthreads-fix.txt is a modified copy of CMakeLists.txt which includes -lpthreads when building the tests/benchmarks.
+Using this instead fixes compilation on some platforms.