memtx: pass engine to memory allocation functions

We need this so that we can force garbage collection when we are short
on memory. There are two such functions: one is used for allocating
index extents, another for allocating tuples. Index allocating function
has an opaque context so we simply reuse it for passing memtx engine to
it. To pass memtx engine to tuple allocating function, we add an opaque
engine specific pointer to tuple_format and set it to memtx_engine for
memtx spaces.