box/schema.func: switch to new module api

Since we have low level module api now we can switch
the box.schema.func code to use it. In particular we
define schema_module structure as a wrapper over the
modules api -- it carries a pointer to general module
structure.

Because of modules reload functionality the schema modules
carry own cache of modules instances. Thus to make overall
code somehow close to modules api we do:

1) cache operations are renamed to cache_find/put/update/del;
2) C functions are switched to use module_func low level api;
3) because low level modules api carry own references we can
   take no explicit reference when calling a function.

Part-of #4642

Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>