Better shutdown for clvmd

Better shutdown for clvmd

'a small step' towards cleaner shutdown sequence.
Normally clvmd doens't care about unreleased memory on exit -
but for valgrind testing it's better to have them cleaned all.

So - few things are left on exit path - this patch starts to remove
just some of them.

1. lvm_thread_fs is made as a thread which could be joined on exit()
2. memory allocated to local_clien_head list is released.
   (this part is somewhat more complex if the proper reaction is
   needed - and as it requires some heavier code moving - it will
   be resolved later.