This commit promotes a few assertions into elog(ERROR) so as to avoid
new data being appended to a segmene file that is not in available
state.  Scans on an AO table do not read segment files that are awaiting
to be dropped.  New data, if inserted in such a segment file, will be
lost forever.

The accompanying isolation2 test demonstrates a bug that hits these
errors.  The test uses a newly added UDF to evict an entry from the
appendonly hash table.  In production, an entry is evicted when the
appendonly hash table is filled (default capacity of 1000 entries).

Note: the bug will be fixed in a separate patch.
Co-authored-by: NAdam Berlin <aberlin@pivotal.io>