Copying and comparing stopped their descent at
basic_json::nesting_depth_limit(), while serializing, hashing and
merging used detail::recursion_depth_limit(). Both were 128, but
nothing kept them equal. The thread-local count now tests against
detail::recursion_depth_limit() as well, and a static_assert keeps
the limit small enough for the byte that holds the count.
Signed-off-by: Niels Lohmann <mail@nlohmann.me>