Full and incremental osdmaps are structured identically and have
identical headers.  Add a helper to decode both "old" (16-bit version,
v6) and "new" (8-bit struct_v+struct_compat+struct_len, v7) osdmap
enconding headers and switch to it.
Signed-off-by: NIlya Dryomov <ilya.dryomov@inktank.com>
Reviewed-by: NAlex Elder <elder@linaro.org>