memtx: free tuples asynchronously when primary index is dropped

When a memtx space is dropped or truncated, we have to unreference all
tuples stored in it. Currently, we do it synchronously, thus blocking
the tx thread. If a space is big, tx thread may remain blocked for
several seconds, which is unacceptable. This patch makes drop/truncate
hand actual work to a background fiber.

Before this patch, drop of a space with 10M 64-byte records took more
than 0.5 seconds. After this patch, it takes less than 1 millisecond.

Closes #3408