This change refactors the chunk index handling to make better use
of standard PostgreSQL catalog information, while removing the
hypertable_index metadata table and associated triggers, including
those on the chunk_index table. The chunk_index table itself is
also simplified.
A benefit of this refactoring is that indexes are no longer
created using string mangling to construct the CREATE INDEX command
for a chunk, based on the string definition of the hypertable
index. Instead, indexes are created in C using proper index-related
internal data structures.
Chunk indexes can now also be renamed and are added in the parent
index tablespace. Changing tablespace on a hypertable index also
recurses to chunks, as expected. Default indexes that are added when
creating a hypertable use the hypertable's tablespace.
Creating Hypertable indexes with the CONCURRENTLY modifier is
currently blocked, due to unclear semantics regarding concurrent
creation over many tables, including how to deal with snapshots.
