librados: move C++ APIs into libradospp

librados: move C++ APIs into libradospp

the goal is to decouple C++ API from C API, and to version them
differently, as they are targeting different consumers.

this allows us to change the C++ API and bumping up its soversion
without requiring consumer to recompile the librados client for
using the new librados. in this way, C++ API can move faster than
C API. for example, if bufferlist interface is changed for better
performance, and this breaks existing API/ABI, we can bump up
the C++ library's soversion, and and the C library's version unchanged
but ship the new librados's C binding. so the librados client linked
against librados's C library will be able to take advantage of
the improvement in C++ library. while the librados client
linked against C++ library won't break at runtime due to unresolved
symbol or changed structure layout.

this is massive change, the genereal idea is to

* split librados.cc into two source files: librados_c.cc and
  librados_cxx.cc, the former for implementing C APIs, the later
  for C++ APIs.
* extract the C++ API in librados into librados-cxx, the library
  name will be libradospp. but we can change it before nautilus
  is released.
* link these librados libraries with static libraries which it
  depends on, so "-Wl,--exclude-libs,ALL" link flags can help
  hide the non-public symbols.
* extract the tests exercising librados' C++ API into a different
  source file named *_cxx.cc. for instance, to move the C++ tests
  in aio.cc into aio_cxx.cc
* extract the shared helper functions which do not use any librados
  or librados-cxx APIs into test_shared{.cc,h}. the "shared" here
  means, *shared* by C++ and C tests.
* extract the test fixtures, i.e., the subclasses of testing::Test,
  for testing C++ APIs into testcase_cxx.cc.
* update qa/workunits/rados/test.sh accordingly to add the splitted
  tests
* update the consumers of librados to link against librados-cxx
  instead, if they are using the C++ API.

Signed-off-by: Kefu Chai <kchai@redhat.com>