memtx: rework background garbage collection procedure

Currently, the engine has not control over yields issued during
asynchronous index destruction. As a result, it can't force gc when
there's not enough memory. To fix that, let's make gc callback stateful:
now it's supposed to free some objects and return true if there's still
more objects to free or false otherwise. Yields are now done by the
memtx engine itself after each gc callback invocation.