box: rework func cache update machinery

Tarantool used to assume that func_new call must not fail and
it used to build a new func object by given definition just on
func cache replace operation. We need to fix it to perform
user-dependent risky actions like Lua function assemble in
further patches.

The replace method is disallowed for _func space because it is
redundant and difficult to maintain in case of functions that
have pre-compiled runtime.

Needed for #4182, #1260