memtx: fix out of memory handling for rtree

When tarantool tries to recover rtree from a snapshot and memtx_memory value
is lower than it has been when the snapshot was created, server suffers from
segmentation fault. This happens because there is no out of memory error
handling in rtree lib. In another words, we do not check the result of
malloc operation.
The execution flow in case of recovery uses different way and the secondary
keys are build in batches. That way has no checks and reservations.
The patch adds memtx_rtree_index_reserve implementation to make sure that any
memory allocation in rtree will fail. Although this gives us no additional
optimization as in case of memtx_tree, the memory reservation prevents
tarantool from segmentation fault. If there is not enough memory to be reserved
server will fail gently with the "Failed to allocate" error message.

Closes #4619