Move PerfCounter from ObjectStore.h to FileStore.h

The anonymous enum which used by FileStore as PerfCounter is only can be
used by FileStore, so we need to drive out ObjectStore.h

Signed-off-by: Haomai Wang <haomaiwang@gmail.com>
Rename filestore_perf_t to objectstore_perf_t

The name "filestore_perf_t" isn't suitable for other backends, so we
need to rename it.

Signed-off-by: Haomai Wang <haomaiwang@gmail.com>