vinyl: fix unwritten mem dropped if ddl

Since commit ba08c2a9 ("vinyl: rotate mem on ddl"), in-memory trees
can be rotated not only by dump/compaction tasks, but also by tx commit,
in case a DDL happened after the mem was created. Dump/compaction tasks
are unprepared for this - they always drop all frozen in-memory trees.
As a result, in case of DDL in the middle of dump/compaction execution,
an in-memory tree can be complete lost along with all its statements. To
fix this, let's count the number of trees we are actually dumping and
only delete them upon task completion.