Implemented action serialization, when operating on same computes (OMS-223)

I decided to implement it differently, than we initially devised. Instead of a marker interface, I've implemented a class-wide lock registry with Twisted deferred-aided action chaining/serialization. This allowed me to generalize the implementation over all of the Compute-specific actions (which I realized by extracting a base class called ComputeAction and renaming old ComputeAction into VComputeAction to better reflect its true purpose and context). 

Still, there are significant gaps in this implementation, as commands and Compute-specific event handlers are not included into this scheme. This needs additional work.

As commands are usually generic and don't know anything about Computes (they are implemented in OMS, not KNOT), it is also possible we don't really need them to lock Computes at all. One notable exception though is `mv`, which definitely needs locking implemented.