memtx: reset read view key_def

Tuples stored in a read view might not have a format, because we
overwrite the tuple format id when we free a tuple. So for anything more
sophisticated than a full-scan, we need to use special comparators that
don't access the tuple format. To address that, let's provide a way to
reset tree and hash key_def if ENABLE_READ_VIEW is defined.

The stub functions simply reset the read view key_def to NULL, because
it isn't supposed to be used in the CE version.

Needed for https://github.com/tarantool/tarantool-ee/issues/197

NO_DOC=internal
NO_TEST=ee
NO_CHANGELOG=internal