memtx: run garbage collection on demand

When a memtx space is dropped or truncated, we delegate freeing tuples
stored in it to a background fiber so as not to block the caller (and tx
thread) for too long. Turns out it doesn't work out well for ephemeral
spaces, which share the destruction code with normal spaces: the problem
is the user might issue a lot of complex SQL SELECT statements that
create a lot of ephemeral spaces and do not yield and hence don't give
the garbage collection fiber a chance to clean up. There's a test that
emulates this, 2.0:test/sql-tap/gh-3083-ephemeral-unref-tuples.test.lua.
For this test to pass, let's run garbage collection procedure on demand,
i.e. when any of memtx allocation functions fails to allocate memory.

Follow-up #3408