Fix race condition when changing node states

To fix race condition we're adding a mechanism of background task
execution in conductor. The conductor will get synchrozed AMQP call,
reserve lock, start the background task and return empty response to
the API. In case when lock cannot be acquired or background tasks pool
is full, the exception is sent back to the API and the task is not
started.

Also the patch adds an ability to control resource locks manually.
This feauture used to release lock in the end of background task.

Change-Id: I4095de2d82058ea5e052531698e67a0947424435
Closes-Bug: #1259910
Merge "Fix race condition when changing node states"