A few fixes to the indexes-only building.

Improve locking behavior in indexes-only mode so we only need one
ACCESS EXCLUSIVE lock per table instead of one per index. Support
multiple --index options. Fix for schema-name parsing.

Patch from Beena Emerson.
Several more fixes for indexes-only mode:

 * Fix error handling in repack_table_indexes() to keep track of which
   indexes were actually rebuilt successfully by pg_repack, so that only
   those indexes will be dropped. Previously, we would issue an error
   message telling the user to use DROP INDEX but then just drop the
   index anyway.
 * DROP INDEX command needs to quote the schema name in case it is
   not simple.
 * missing CLEARPGRES() in repack_all_indexes()