include/cpp-btree: use the same type when allocate/deallocate

include/cpp-btree: use the same type when allocate/deallocate

btree_set<> by default uses `std::allocator<Key>`, and btree_map by
default uses `std::allocator<std::pair<Key, Value>>`.

before this change, btree uses the allocator directly for allocating n
elements where element is `Key` or `std::pair<Key, Value>` respectively,
while "n" is actually supposed to be the number of bytes used by each
node which is being allocated.
but, what we need to allocate is actually a "node_type" for holding
multiple slots, and each slot holds an element. in addition to the
slots, a node also keeps track of metadata for btree itself. in short,
what we allocate now is (in bytes):

  alignof(sizeof(node_type)) * sizeof(element)

but what we should allocate is (in bytes):

  alignof(sizeof(node_type))

in this change:

* always rebind the allocator to the correct aligned type with given
  alignment
* extract the allocator related helpers into a template class

Signed-off-by: Kefu Chai <kchai@redhat.com>