connection: Rename wl_buffer

struct wl_buffer has other meaning in wayland, thus making this a pretty
confusing structure name. Function names like wl_buffer_put() just
compound the confusion.

Rename the struct and the associated functions (none of which are called
from outside this file anyway). The struct retains a wl_ prefix, as is
the custom for wayland internal data structures. The function names
have not retained this prefix, as we have many static function that
aren't prefixed.

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>