mgr / volumes: maintain connection pool for fs volumes

mgr / volumes: maintain connection pool for fs volumes

Right now every [sub]volume call does a connect/disconnect to the
cephfs filesystem. This is unnecessary and can be optimized by
caching the filesystem handle in a connection pool and (re)using
the handle for subsequent [sub]volume operations.

This would be useful for implementing features such as purge queue
for asynchronous subvolume deletes.

Signed-off-by: Venky Shankar <vshankar@redhat.com>