Rename _is_writable() with _is_exclusive because the writability does not depend
on the amount of references to the memory object anymore.
Add accessor macros for the memory flags.
Rename the GstBuffer _peek_memory() method to _get_memory() and return a
reference to the memory now that we can do this without affecting writability
of the memory object. Make it possible to also make this function merge the
buffer memory.
Add methods to replace memory in a buffer. Make some convience macros for the
buffer memory functions.
Fix unit tests.