Allow user to control lifetime of Buffer object

[ltaylor16]

Many Sidre users will never interact with Buffer objects directly. They are created as part of creating a View and released when there are no Views associated with the Buffer.

However, there are cases where the user cares about the lifetime of the Buffer. For example, they create a Buffer with createBuffer then create several Views which use consecutive blocks in the Buffer. When the Views are destroyed the user has no convenient way to preserve the Buffer.

A field should be added to the Buffer object to define ownership. When createBuffer is called by the user, ownership is set to the user. Calls to createBuffer internal to the library should set ownership to the library. Then when the last View is removed from a Buffer the View will no longer be deallocated if the user owns the Buffer.

If the Buffer is created by the library, then extracted by the user via getBuffer and used to create another View, the old semantics should still apply. This keeps the ownership model simple since only createBuffer defines user ownership.

Issue #674 should include a flag to force destruction of user owned Buffers.

When the DataStore destructor is called, all Buffers should continue to be destroyed.