client: add mount/initialize states support and convert to RWRef

client: add mount/initialize states support and convert to RWRef

This will help us to break the big client_lock. Such as if we want
to add one separate lock for each inode, it will work like:

inode_lock.lock();
...
client_lock.lock();

client_lock.unlock();
...
inode_lock.unlock();

So we must get rid of the client_lock for all the "readers", which
must hold the client_lock from the beginning until the end.

Fixes: https://tracker.ceph.com/issues/46649
Signed-off-by: Xiubo Li <xiubli@redhat.com>