api3: Allow lcb_install_callback3 to accept NULL

Unlike the older v2 callbacks, where a `NULL` parameter means to return
the current callback, in the v3 callbacks, a `NULL` simply means to
utilize the `LCB_CALLBACK_DEFAULT` callback.

This also adds an `lcb_get_callback3()` function, by which it is
possible to explicitly obtain the older callback (without changing the
current one); something not possible with this version of
install_callback3()

Change-Id: If8b7c8e03a2ef9daed7343f7bb2e82b791543325
Reviewed-on: http://review.couchbase.org/42474
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>