Add shareable base class

[ehpor]

This PR adds a base class for all objects that are shared between processes. A shareable object has a shared state and a local state. The shared state is assumed to be in shared memory and is accessed by the shareable object using a pointer. The local state is internal to the shareable object. Shareable objects have Create() and Open() methods that take a pointer to that shared state to initialize or open. Note: this base class provides no methods for actually sharing this information between processes. That's the job of a SharedMemory or other similar classes. It just formalizes the size and open methods, and provides an enum for all shareable types.

This PR also adds an Event class that combines all other Event implementations into one single class. An Event allows you to wait using multiple methods, choosable at runtime. This will be useful for testing comparing synchronization methods in practice rather than only during tests (like I've been doing so far).

The data stream is also changed to use two shared memory blocks per stream. The first one is for header information, communicating where each data frame is in the buffer. The second is the buffer where the actual data is stored. This allows us to support GPU shared memory in the future (since the header information should not be stored in GPU shared memory but rather on CPU shared memory. Therefore, two separate shared memory buffers is required.

Risk to existing testbeds: no risk. Everything should be refactor only, all modifications are C++ only with no side-effects leaking into Python code.

[ehpor]

@raphaelpclt @ivalaginja This is ready for your review now. I'll do some more testing of edge cases, but I do not see this changing anything significantly other than 1-2 lines here and there, if any.