There always was a one-to-one mapping between dict_tree_t and dict_index_t.
This saves 6 machine words per B-tree index in the data dictionary
cache plus the memory allocation overhead. We save one mem_heap_t
object per index (15 machine words). Considering the internal
fragmentation of the buddy allocator in mem_area_alloc(), this should
save 32 machine words per index (128 bytes on 32-bit systems and 256
bytes on 64-bit systems). (Bug #20877)
struct dict_tree_struct, dict_tree_t: Remove.
struct dict_index_struct: Add page and lock.
dict_tree_create(): Remove. Replace the invocation with
assignment to index->page and a call to rw_lock_create(&index->lock).
dict_tree_free(): Remove. Replace the invocation wtih
a call to rw_lock_free(&index->lock).
dict_index_get_tree(): Remove.
dict_tree_get_space_reserve(): Rename to dict_index_get_space_reserve()
and remove the parameter, which was unused.
btr_level_list_remove(): Remove the unused parameter "tree".
Replace the occurrences of "tree" with "index" in names of variables,
functions and data types, e.g. "dict_tree_t tree" becomes
"dict_index_t index". Remove local variables "tree" or "index" of
functions that needed both "tree" and "index".
