Change the compute polling local cache to be mutex
Change the compute polling local cache to be mutex

When configure multiple polling tasks with different intervals, the local
cache may be concurrently accessed by multiple threads, that may cause
data inconsistency. This change add a threading lock to make the local
cache to be mutex.

Change-Id: I07512928a27f4f537394200a989e2f6c85528dab
Closes-Bug: #1682054

openstack/ceilometer,openstack/ceilometer