mds: use mempool for cache objects

mds: use mempool for cache objects

The purpose of this is to allow us to track memory usage by cached objects so
we can limit cache size based on memory available/allocated to the MDS.

This commit is a first step: it adds CInode, CDir, and CDentry to the mempool
but not all of the containers in these classes (e.g. std::map). However,
MDSCacheObject has been changed to allocate its containers through the mempool
by converting compact_* containers to the std versions offered through mempool
via the new alloc_ptr.

(A compact_* class simply wraps a pointer to the std:: version to reduce memory
usage of an object when the container is only occasionally used. The alloc_ptr
allows us to achieve the same thing explicitly with only a little handholding:
when all entries in the wrapped container are deleted, the caller must call
alloc_ptr.release().)

Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>