compaction_state shouldn't be moved once emplaced. moving it could
theoretically cause task's gate holder to have a dangling pointer to
compaction_state's gate, but turns out gate's move ctor will actually
fail under this assertion:
assert(!_count && "gate reassigned with outstanding requests");
Cannot happen today, but let's make it more future proof.
Signed-off-by: Raphael S. Carvalho <raphaelsc@scylladb.com>
Closes#12167
d61b4f9dfb