salad: allow to pass matras statistics to LIGHT, BPS tree, R-tree

Simply forward the new matras_stats argument to matras_create().
Currently, it's only used in tests.

While we are at it:
 - Drop custom alloc/free func types in favor of matras types since
   now container constructors depend on matras_stats anyway.
 - Rearrange arguments order to group allocator-related arguments
   together.
 - Drop rtree_init return value (it's always 0).

Needed for https://github.com/tarantool/tarantool-ee/issues/143

NO_DOC=refactoring
NO_CHANGELOG=refactoring