compressor: put Compressor into TOPNSPC namespace

compressor: put Compressor into TOPNSPC namespace

we want to add the support for on-the-wire compression to msgr v2, and
this feature will be shared by classic osd and crimson. but
Compressor.cc is also used by bluestore which is linked against by
crimson also. Compressor depends on CephContext which has two different
implementation in two namespaces for classic osd and crison.

so to avoid violating ODR, we should also put Compressor into two
different namespaces so it can be shared by alienstore and crimson in
the same executable.

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