cmake: turn libcommon into a shared library

cmake: turn libcommon into a shared library

prior to this change, libcommon is a convenient library which gets
linked into librados, librbd and libcephfs and all ceph executables.
this incurs some problems:
 - double dose of libcommon in memory space and HDD: waste of memory
   and disk space.
 - if an application links to two libraries including libcommon at the
   same time. take librados and libcephfs as an example, they could
   interfere with each other by changing the other guy's status.
after this change, libcommon is tuned into a shared library and
renamed to libceph-common. it will be installed into $prefix/lib/ceph,
and packaged in librados2.

ceph.spec.in,debian/librados2.install: package libceph-common in
  librados2.
CMakeLists.txt:
  - do not link against libboost-* if not necessary.
  - s/common/ceph-common/g
  - install libceph-common into $prefix/lib/ceph
  - set rpath to $prefix/lib/ceph
  - link against ceph-common if an executable needs access to non public
    symbols in ceph.

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