lib: add "borrow" functions where "get" functions exist

lib: add "borrow" functions where "get" functions exist

The new bt_*_borrow_*() functions return a "borrowed reference" instead
of a new reference like the bt_*_get_*() functions return.

Use the "borrow" functions as much as possible internally.

This can improve performance, especially for objects which are obtained
often, and for a short time, in the fast path (fields, events, packets,
notifications, some metadata objects), as it avoids one bt_get() and one
bt_put() call for each function call.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>